Chapter 2 The Insects McShaffrey Draft 1 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 2 of 20 What is an insect? Insects, despite their diversity of forms, are considered to be a single group because they share several characteristics that, according to the experts, have evolved only once and then were passed on through time as the insects diversified. Many of these distinctions are technical, but several are easily seen and I will focus on these. I m also going to toss in a few scientific terms here and there to help you gain familiarity with them so you ll be ready when you consult more advanced texts on insects. First of all, insects are Arthropods. This is a larger grouping that includes insects, crabs, lobsters, shrimp, spiders, scorpions, mites, millipedes, and a host of other organisms you ve probably never heard of. Arthropods themselves share a few characteristics which are passed on to insects as well. The name Arthropod literally means jointed foot, and you ve no doubt encountered the root terms in your daily life, perhaps when having arthroscopic surgery (literally scoping out the joint or perhaps if you ve had to visit a podiatrist, who, of course, is a doctor of the foot. In Arthropods, the joints are found on most of the appendages, including the antennae. Look at other organisms, such as an octopus, and you ll notice that the appendages are flexible, but not jointed. Arthropods also have an exoskeleton; the hard outer covering also acts as support for the body. An arthropod s body itself is jointed, with the segments joined together by flexible sections. A pseudoscorpion, one of the non-insect Arthropods. This non-stinging scorpion relative is tiny; note the pores of my painted cement block basement wall. Most of the Arthropods are classified into subgroups based in large part on how the segments are arranged and fused, and how the legs are arranged. The original arthropods probably had legs on each segment, and each segment and its legs were pretty much like any other segment. Over time, the segments up front started to specialize for feeding and the legs on those segments were modified into what we now call mouthparts. The front segments fused into what we call the head. Other segments fused into a thorax and still others into the abdomen, although this varies by group, and in fact is one of the ways we distinguish the groups. Spiders, for instance, have a cephalothorax (fused head and thorax) and abdomen, while millipedes are much like the ancestral arthropods in having most of the body composed of very similar segments (aside from the modified segments of the head). McShaffrey Draft 2 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 3 of 20 This brings us to the insects. They have 3 body regions (tagmata) the head, focused on sensory structures and feeding, the thorax, concerned with locomotion, and the abdomen, used primarily for reproduction. The head bears 4 pairs of modified appendages used as mouthparts and a single pair of antennae. The insect thorax has 3 pairs of jointed legs (a total of 6); although in some species or life stages (such as a fly maggot) the legs are not present. The thorax has 3 segments (each bearing a pair of legs); the second and third segments of the thorax each may or may not bear a pair of wings. The abdomen is ten segments long, usually does not have appendages (if present they are never jointed), and usually bears reproductive structures at its tip. The 3 major body regions of an insect (a lubber grasshopper, in this case) Insects like this wasp undergo a radical transition from the larval stage (bottom) through the pupa (middle) to the adult (top) These characteristics will always identify an insect, although a few oddballs may give you some grief. Maggots, as mentioned, are legless, as are some bee and wasp larvae. Any animal with wings that is not a bird, bat or pterosaur is an insect, and if it has 4 wings it definitely is an insect. If it walks on more than 6 legs it s not an insect. Now you already knew all of that, I m sure. You may not be as aware of how insects are divided up into various groups, but I ll bet that if presented with a number of common specimens you d probably be able to place them into the appropriate group. Most of us can easily identify a fly, a beetle, or a butterfly (although I ve seen flies in with bees in museums, presumably placed there by an expert!). This next section will introduce you to the anatomy that is used to separate and identify the various groups of insects. I ll try to keep the jargon and terms to a minimum, but you do want to learn something, so I ll put some of the more important terms in. If you want to go into greater depth, you might want to consult an entomology textbook; in my opinion you can t do much better than Borror and DeLong's Introduction to the Study of Insects, which nowadays is being revised by Norman Johnson and Charles Triplehorn. McShaffrey Draft 3 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 4 of 20 Insect Anatomy Body Plan Before getting to the anatomy proper, a few basic terms that define directions on the organism are needed. These terms are used by zoologists to describe all types of animals. Hold an insect with the head to your left. The top or back surface of the animal is dorsal; the belly is ventral. Thus we say that the wings are attached dorsally, or that the legs are ventral. The head is at the anterior or cranial end of the body; the opposite end is posterior or caudal. If you turn the insect so that you are looking straight into the face of the animal, you are looking at the anterior end. From this position, imagine a plane running down the middle of the organism, dividing it into right and left halves. This line defines the medial, or the middle portion of the body. Moving away from the median to either the left or the right means moving laterally. Now, consider one of the legs. It is attached to the body at its basal point; on any appendage such as a leg or a wing moving towards the point of attachment is moving proximally and moving away from the body means moving distally. The end of an appendage, such as the claw on the end of a leg, is said to be apical. Thus basal and apical are antonyms and are absolute terms; where proximal and distal are also antonyms but in this case they are relative. On your body your wrist is distal to your elbow, but the wrist is proximal to the hand. McShaffrey Draft 4 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 5 of 20 In most insects, the body segments can be thought of as a 4-sided box. The top of the box would be a plate called the tergum (plural terga); the bottom of the box a plate called the sternum (plural sterna), and the side plates would be called pleura. A tergum on the thorax of the body is called a notum. The 3 thoracic segments are called (from front to back) the prothorax, mesothorax, and metathorax; the dorsal plates covering these segments are the pronotum, mesonotun and metanotum, respectively. In this diagram, the arrow for the tergum is pointed specifically to the pronotum and the arrow for the sternum is pointed specifically to the prosternum. Several other terms apply to all regions of the body and will be of use to you as you examine specimens. A carina is a ridge in the exoskeleton; a suture is a groove in the exoskeleton. Sutures are found where segments meet, and in other places where the exoskeleton folds internally to provide strength. A seta (plural setae) is a socketed hair projecting from the body; a spine is an unsocketed projection of the exoskeleton; a spur is a multicellular, socketed projection. While the exoskeleton is usually hard and shiny, there is an important exception. Pruinosity refers to an extrusion of a powdery, waxy substance over the surface of the exoskeleton. This may change the apparent color of the body to white or powdery blue. Pruinosity often increases with age, and may obscure color patterns. McShaffrey Draft 5 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 6 of 20 The Head We ll start at the bottom of the head and work upwards. The most posterior of the mouthparts is the lower lip, or labium. It forms the back of the mouth cavity. Just in front of the labium are the paired maxillae, which function as accessory jaws; these move in and out in a lateral direction. Most of the work of the mouth is done by the main jaws, or mandibles, which lie in front of the maxillae. The mandibles are large, heavily muscled, and bear teeth on their inner edges. These jaws tear up the prey so that it can be swallowed. Finally, the labrum, or upper lip, covers the mouth from the front. I should mention at this point that some insects such as mosquitoes and stink bugs have highly modified mouthparts. The same structures are there, it s just that they don t resemble the traditional mouthparts of insects like grasshoppers. One example of these highly modified mouthparts is the beak, or rostrum, of the Box Elder Bug pictured here. Looking at the insect face on, we continue upwards from the labrum. The labrum is attached to the head at a plate called the clypeus; above the clypeus is a region of the head called the frons. The suture separating the frons from the clypeus is the frontoclypeal suture. The clypeus itself can be split into an upper postclypeus and a lower anteclypeus. Together, we call the labrum, the clypeus, and the lower, forward facing part of the frons the face. The upper surface of the frons faces dorsally and is sometimes called the forehead; it may be a different color from the rest of the frons. McShaffrey Draft 6 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 7 of 20 On most specimens, one cannot help but notice the large compound eyes, which may cover most of the head of an adult insect (larval compound eyes may be absent or much smaller). The compound eyes of a dragonfly, for instance, each may have 28,000 facets; each facet forms an individual image. Insects see the world differently than we do. Insects in general are sensitive to the polarization of light in the sky, and can sense the sun s position (even on a cloudy day) from this information. Likewise, they can see ultraviolet light that we cannot. Most also have 3 simple eyes, or ocelli (ocellus, sing.), located on the vertex the top of the head right in front of the compound eyes. The simple eyes sense light and dark, but do not form images (as far as we know). There may be a raised region, or postocellar crest behind the ocelli. The posterior portion of the top of the head is known as the occiput; if the compound eyes meet, they separate the occiput from the vertex. The antennae of insects range from small and bristle-like in the Odonata to huge elaborate structures in other groups. In some insects, such as male moths (pictured), the antennae are elaborate and can identify and track chemicals at fantastically small concentrations. Other insects, such as male midges, can hear the wingbeat of females of their species with their own elaborate antennae. The Thorax The thorax of an insect lies directly behind the head and bears the wings and the legs. As mentioned above, the three components of the thorax are the prothorax, the mesothorax, and the metathorax, in order from front to back. The prothorax bears the front pair of legs. The mesothorax and metathorax each bear a pair of legs, and may or may not each bear a pair of wings. Most adult insects will have a pair of wings on both of these segments. All larval insects as well as some adults are wingless. If an insect has only one pair of wings, they are usually on the mesothorax, as is the case with the true flies or Diptera (the name itself means 2-winged. A few insects, however, have but one pair of wings and those are located on the metathorax. McShaffrey Draft 7 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 8 of 20 Wings Perhaps the most significant structures on the thorax, at least from the standpoint of identification, are the wings. The wings are composed largely of non-living chitinous cuticle; this material makes up most of the membranous portions of the wings. The living components of the wing are confined to the veins, which are actually more complicated than the simple tube the name vein implies. In addition to carrying blood, the veins also contain tracheae (air tubes), and nerves. These nerves are connected to hairs on the surface of the wings; the hairs sense air movement over the wing and help the insect to regulate its flight. The wing veins of this ant lion adult form an intricate net that reminded someone of interconnected nerves, hence the name Neuroptera or Nerve Wing for the order. The pattern of veins, as well as the size and shape of the cells bounded by the veins in the wing provide a fingerprint that is unique for many genera, and even some species of insects. While many species can be identified without resorting to the wings, unfortunately, it is impossible to identify some species without detailed examination of wing venation. Complicating the situation is that experts think that wings have arisen several times among the insects and thus the veins of the wings in certain groups, such as dragonflies, do not match up well with the names in other groups, although that hasn t stopped other experts from naming them as if they did! Except for a few obvious cases, I won t use wing venation characters in this book. McShaffrey Draft 8 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 9 of 20 Legs The legs of insects often tell us a lot about its lifestyle. Some flying insects merely use the legs to perch. Other insects may have running (cursorial, like the little thing that runs around your computer screen) or jumping (saltatorial), or digging (fossorial, fossils are foujnd underground), or raptorial (used to grasp, as in a preying mantis) legs. In all cases, the basal segment is called the coxa, the next distal segment is the trochanter. Following these segments are the much larger femur and tibia; together these two segments comprise most of the length of the legs. Note also that the names of these segments match the names of the corresponding bones in the human leg. Attached to the tibia are several (1 to 5) tarsal segments; the last tarsal segment usually bears one or two tarsal claws. The Abdomen Next to the wings, the abdomen bears some of the most useful characteristics for the identification of insects, especially in males. This is in part due to the fact that the male genitalia are located there. These structures are often anatomically modified in groups lacking behavioral courtship displays, probably to ensure that males and females of a particular species mate with each other and not individuals of other species. However, interspecific pairings do occur. Interspecific matings are probably mostly infertile, and thus it seems logical that any mechanism that would prevent such pairings would be selected for. McShaffrey Draft 9 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 10 of 20 Most insects have 10 abdominal segments. These are numbered from 1 to 10; starting with segment 1 nearest the junction with the thorax and moving to segment 10 at the tip of the abdomen. Segment 1 is attached to the thorax while the terminal appendages are borne on the end of segment 10. These terminal appendages may include 2 dorsal cerci (superior appendages) which are used for a variety of purposes. For instance, the cerci of cockroaches are very sensitive if another insect comes up behind a cockroach the cerci sense it and are wired into some of the fastest nerves in the animal kingdom, triggering a quick escape. They are even sensitive to air movements, as anyone who has tried to swat a cockroach knows only too well. The terminal segments of the female abdomen are fairly simple compared to most males which often have elaborate genitalia. The complexity of the female reproductive system are largely internal, but externally there may be structures to help with egg laying. Most obvious is an ovipositor; this is used by some species to lay eggs in the ground, rotting wood, plant or animal tissue (ouch!) or in other substrates. The ovipositor may be very large; ichneumonid wasps (right) have a threadlike ovipositor about 4 inches long that can be worked through the wood of a dead tree to sting and insert eggs into a beetle grub several inches below the surface. McShaffrey Draft 10 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 11 of 20 Larval Insects The insect life cycle, which we ll explore in detail shortly, is usually divide up into several stages a single egg stage, a number of immature stages, and finally the adult or imago stage. Different groups of insects vary in the way they develop through the immature stages and make the transition to the final, adult stage. These differences are useful in classifying insects and crucial in recognizing and identifying them. Now, some nomenclature. Many different words are used for immature insects, and purists often insist on using the proper term for each type of immature insect. That might be necessary in an entomology class, but not here. Still, it s important to recognize some of the terms. Traditionally, the term nymph was used for an immature insect which resembled the adult in most ways except that it lacked fully developed wings and mature genitalia. Unless the nymph in question was aquatic, in which case it was called a naiad, unless you were a fly-fisherman (or woman) in which case you probably called all immature insects nymphs. Confused? It gets worse. The term larvae was used to describe insects which make a more dramatic change from the immature to the adult stage, as in the case of a caterpillar transforming into a butterfly. Often these dramatic changes include a transitional stage which just about everybody calls a pupa, unless it s a butterfly and thus called a chrysalis. Further, particular groups have specific names for their immature stages maggots for flies, caterpillars for butterflies and moths, grubs for beetles, bees and wasps, etc. Insect Life Stages: 1. Fly, adult and eggs. 2. Wasp, adult, pupa, larva. 3. Mosquito, pupa. 4. Box Elder Bug, adult and eggs. 5. Catalpa Worm, caterpillar. 5. Milkweed Bug, adult and nymphs. 6. Dog-day Cicada, adult emerging from larval skin. 8. Mayfly, naiad. McShaffrey Draft 11 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 12 of 20 I m a professional, folks. Don t try these terms at home. I herby give you permission to call all the immature stages larvae. Unless they are pupae or eggs. Or live in water. Or the month ends in a G. Okay larvae have the same body regions, appendages, etc. as the adults with a few exceptions. First, no larvae have functional wings. Second, there are two major groups or types of larvae, those which undergo a dramatic change and those which don t. The ones that don t will look a lot like the adults except that the wings will be encased in enclosures referred to as wing pads. The larvae that do make the dramatic change will go through a pupal stage and the larval stage may not look at all like the adult. A case in point are caterpillars they have no wings or wing pads, they have regular chewing mouthparts unlike the highly modified nectar straws of the adults, and in addition to the 6 jointed legs characteristic of the insects they may have several pairs of prolegs fleshy legs that do the work of legs but which differ enough that they are not considered to be true legs and thus don t count when counting legs. Still other larvae, such as fly maggots (left), do not have any jointed legs at all, and may even lack prolegs. That s the basics of insect anatomy. We ll go into more detail and introduce some additional structures as well as exceptions to virtually everything I ve said as we study each group of insect later in the book. Let s turn our attention now to the life cycle of insects. An adult Box Elder Bug with young (nymphs). The nymphs resemble the adults except that they lack wings (although older ones have wing pads). Thus, they are said to have incomplete or gradual metamorphosis (hemimetabolous) as opposed to the complete, sudden or holometabolous metamorphosis of insects like the maggots. McShaffrey Draft 12 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 13 of 20 Life cycle A Bembix wasp has stung a stink bug and is placing the paralyzed insect in an underground nest where it s larva will feed on the still-living bug. Let us begin our review of the life cycle with egg-laying, or oviposition. This is a particularly crucial stage, as adult insects really ony have 2 jobs to mate, and to lay the eggs in a place where the hatching larvae will find protection and food. Adult females use a variety of cues to find the right place to lay their eggs. Butterflies, for instance, need to home in on the right type of plant, as many species are specifically adapted to overcome the chemical defenses of only a single species of plant. Visual and chemical cues play a role in helping the butterfly identify the proper plant. Insects with more generalized diets might not be as choosy in terms of the vegetation, but other factors might be critical. Crickets and grasshoppers, for instance, will look for soil with the right texture and moisture content. Aquatic insects might search out bodies of water with the proper conditions, such as water flow or bottom type. Some aquatic insects lay their eggs on dry land in areas prone to seasonal flooding; yet others lay them in plants growing over streams and lakes so that the larvae can fall into the water after hatching. Insect eggs may be laid individually or in groups. Sometimes the eggs are actually inserted into a plant an example would be the periodical cicadas whose long ovipositors slash into the tender stems of selected trees, or into an animal. For instance, many wasps sting their prey to paralyze it, and the eggs too may be injected to hatch within the paralyzed host and consume it from within. Speaking of wasps, social wasps, bees, ants, termites and other social insects may lay their eggs in constructed nests and care for the larvae. There is great variation in the number of eggs a female can produce; some lay but a few eggs while others may produce tens of thousands. Many female insects will only lay eggs once, then die; others may produce eggs over their entire adult lifespan (which in bees, termites, ants and other social insects may be several years). McShaffrey Draft 13 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 14 of 20 Often, mistakes are made in egg laying. One species of dragonfly which normally lays its eggs in temporary ponds created after heavy rains is also noted for cruising parking lots and laying its eggs on shiny cars (hence the reason I rarely wash may car!). One of the best pictures I have of insect eggs came about when a Box Elder Bug decided to lay its eggs on the side of a white college van on a field trip (right). Once laid, the eggs may hatch immediately, or continue in the egg stage for some period of time. Actually, some eggs hatch within the mother and are actually laid as larvae; a common example would be the larvae of various flies that live in cow manure. So anxious are they to get to work and get the freshest nutrients out of the dung that these flies will hover behind the cattle and flip the larvae into the manure stream as it is deposited. In the temperate areas of the world where there are pronounced seasons, however, egg hatching is typically delayed (the delay is called diapause) until weather conditions are favorable and food is abundant. Thus, eggs laid in the fall may overwinter until spring warmth or rains trigger hatching the following year. If the adults jobs are to mate and lay eggs, it is up to the larvae to eat and grow. Once hatched from the egg they immediately go to work eating. The exoskeleton of insects protects these larvae, but it also is a hindrance at this time as it prevents much in the way of growth. Periodically it must be shed. The insects mentioned above whose larvae resemble the adults typically have what is known as simple, gradual or incomplete metamorphosis in which the shedding of the exoskeleton the molt produces an organism which looks like its predecessor only a bit larger. These organisms typically have many such molts, perhaps dozens over a larval lifespan that may reach several years, but which usually takes only a few weeks for most insects. The insects such as caterpillars which have the more dramatic complete metamorphosis usually have many fewer molts as the softer exoskeleton of a caterpillar or maggot can accommodate more stretching and growth before it must be shed. Typically, complete metamorphosis is accomplished with 5 or so larval molts, plus a molt to the pupal stage and a final molt to the adult stage. After every molt the emerging insect is soft and easily damaged. They quickly swallow air to increase their body size and stretch to their maximum potential. At this point, enzymes are released to cure and harden the outer skin (often giving it its color as well). Once hardened the insect will fill in the body with additional tissue, usually fats and muscle as it feeds and grows. McShaffrey Draft 14 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 15 of 20 An adult midge emerging from its pupal skin. The insect has floated to the surface of the water and is emerging from a position where it lays on its back. The long legs have been pulled out but not yet hardened; the wings are still trapped in the pupal skin. This is one of the mishaps that can occur in this complicated process; normally the midge adult emerges right-side up, standing on the floating pupal skin which forms a raft. Under normal circumstances the midge emerges in seconds; any longer and it is likely to become fish food as this poor girl is doomed to be. A little bit of terminology needs to be introduced here. The time between molts is referred to as an instar; typically insects with complete metamorphosis have fewer instars than those with the incomplete metamorphosis. A hormone in the larval insect determines what type of molt will take place. This hormone, juvenile hormone, when present in high concentrations prevents the development of adult characteristics such as wings or genitalia. In gradual (incomplete) metamorphosis the larva s brain produces less and less of the juvenile hormone with each molt, so that eventually the adult characteristics emerge. Larvae with complete metamorphosis ramp down the production of juvenile hormone more suddenly; after staying at relatively high levels it drops below a critical point at the molt to the pupal stage and stays low during the transformation to the adult. In the insects with this sudden metamorphosis there is a distinct form known as the pupa. Many books treat this stage as a resting stage since it usually does not move about and feed. However, nothing could be further from the truth, for within the pupal body huge transformations are occurring as the adult features emerge from a body that was extremely simple in form. Usually pupation takes place within a hardened larval skin, a cocoon of silk spun by the larva as its last act, or in some hidden place (or a combination of all 3). This, along with some chemical defenses, serves to protect the relatively immobile pupae from predation or parasitism. Other pupae, however, may be quite active. Mosquito pupae (right), for instance, swim about actively when disturbed (although they are no longer feeding). Other pupae wriggle violently when disturbed; they may also be protected by spines and other structures that pinch the unwary predator as the pupa thrashes about. McShaffrey Draft 15 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 16 of 20 Eventually the pupal form comes to an end with ecdysis, the shedding of the pupal skin (or the last larval skin) and the emergence of the winged (in most cases) adult insect. This is usually the most complicated of all the molts, as it is not a trivial task to extract the wings from their cases in the larva or pupa. With the wings free of the last instar s skin the insect pumps air and fluids into the veins of the wings to expand them and allow them to harden. In many insects this process may take hours, during which time they are extremely vulnerable to predation as it is difficult to fly on soft wings; for this reason the final ecdysis often takes place at night. Other insects, including mosquitoes, are able to fly almost immediately after bursting from the pupal skin. Perhaps because the final molt and extraction of the wings is so difficult, only one primitive group, the mayflies, molt after reaching the winged stage. A Dog-Day Cicada making its last molt. This cicada is luckier than the midge on the previous page; its molt is going well and the soft wings will soon expand and harden. The hard larval skin (with the wing pads clearly visible) will be left clinging to the branch after the adult flies away. This is an insect with incomplete or gradual metamorphosis. Once again, the adult stage s job is dispersal and reproduction. During this phase the adult must find a mate and fly to a place suitable for laying eggs. It is usually at this stage that new habitats are colonized. In many species, all of the nutrients needed to produce eggs were procured during the larval stage, so the adult insect is not as concerned with feeding, although many, like butterflies, will sip some nectar to provide energy to fly. Some adult insects don t even have functioning mouthparts! Others will obtain additional resources to make eggs or to make more eggs. Perhaps the most obvious (and obnoxious) example are the mosquitoes; the adult females take blood meals to obtain the proteins needed to produce eggs (the larvae often live in a protein-poor environment). Adult male mosquitoes do not feed other than to sip some nectar. Likewise, early in the year yellowjacket colonies are seeking protein to help rear the young in the hive. They scavenge insects, cut them up, and return to the hive with them. They are not above visiting a picnic, where they hang out near the meat. Late in the season, with no additional young to rear, the hive simply needs sugars to keep the adults going and the workers will visit rotting fruits for the sugars contained there. At a late season picnic or tailgate they tend to go for the colas and beers, again seeking sugars and making a powerful argument for not drinking out of aluminum cans, which may conceal the worker wasps within until it is too late! McShaffrey Draft 16 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 17 of 20 Dispersal would seem to be fairly straightforward for species with wings. Even mosquitoes can cover 25 miles in a single evening (aided by favorable breezes). Butterflies, such as the Monarch, are famous for their long migrations; and many species of dragonflies are now known to migrate long distances as well. One species follows weather fronts, laying its eggs in the temporary ponds formed by the rain that accompanies the fronts. In the United States, the diminutive potato leafhopper similarly rides on storm fronts and moves northeast in this fashion every year from its wintering ground in the southwest. Stream insects tend to fly upstream as adults, perhaps to compensate for the downstream movement of the larvae in the water, and the adults also tend to spread out to new streams as well. Most flying insects tend to seek out a good place for the young to grow and develop. Often these places are widely separated and not predicable as to when and where they will be found. For instance, flies and beetles that feed on carrion must track down the dead animal (usually an easy task given the fact that dead animals tend to smell!). Other insects require that the eggs be laid on a particular species of plant, and chemical and visual cues usually play an important role here. Parasitoids, insects that develop living inside other insects, have a particularly tough job as they must locate their tiny, moving prey. Often they get close by homing in on the same cues that attract their prey to a good site for egg laying (oviposition). For instance, a parasitoid of the Catalpa Worm (a caterpillar that feeds on Catalpa leaves) will itself be attracted to Catalpa trees, where it is most likely to find the Catalpa Worms (below). A wasp that lays its eggs in wood-boring beetle larvae faces a doubly difficult task as the grubs are boring through the wood several inches down. The wasp located the grub by sensing the vibrations that the grub makes as its mandibles tear through the wood fibers. The vibrations are sensed by the feet, and once the grub is located a long, thin ovipositor (stinger) is threaded through the solid wood to sting the grub and inject the egg, which will hatch out and consume the grub from within. McShaffrey Draft 17 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 18 of 20 Left, the wasp larvae as the emerge from their host and spin their cocoons. Life and Death for the Catalpa Hornworm. Above, young first-instar caterpillars hatch from eggs laid by their mother on the underside of a Catalpa leaf. Below, a 5 th -instar larvae near death. Right, a newly emerged wasp climbs over the cocoons; the caterpillar has dried to a husk. Below, a pair of the adult parasites. Dozens of small wasp larvae, injected as eggs into its body, have completed their development and bored out of the caterpillar s body. On its surface, they have spun silk cocoons in which they will pupate. After hatching, the adult wasps will seek out new caterpillars. McShaffrey Draft 18 Last printed 1/12/2007 9:14 PM
Entomological Light & Magic Insect Photography in a Digital Age Page 19 of 20 Some species do not have wings, or if they do, just don t use them. These species usually have reliable food sources that are both predictable and stable. Any dispersal is handled by simply crawling to the new site. Some species use a double strategy. Water Striders, (sometimes called Jesus Bugs for their ability to walk on water), often have both winged and non-winged forms. Early in the year, the non-winged forms predominate. By not putting resources into wings (and the muscles to power them), they can put more energy into eggs, and build up a population quickly. Generations produced later in the season will often have more winged individuals (right) who can fly off to colonize new habitats (and find good overwintering sites). Fleas don t need wings; they spread from host to host as the hosts come together to mate or otherwise interact. The majority of ants don t have wings, but it is important to note that the reproductive stages of ants do have wings and once again it is these stages that handle the reproduction and dispersal. In some insects only the males are winged; this allows females to remain at a good site for oviposition and feeding while the males handle genetic dispersal, mixing up the genes. The females, by not developing wings, once again have more energy to put into producing eggs and thus produce more offspring than winged females would. McShaffrey Draft 19 Last printed 1/12/2007 9:14 PM
Chapter 2 The Insects Page 20 of 20 Chapter Heading: A Soldier Fly (Diptera: Syrphidae) laying eggs on a blade of grass. One of the reasons there are so many insects is that they are so prolific. This single female has produced hundreds of eggs. McShaffrey Draft 20 Last printed 1/12/2007 9:14 PM