Evolutionary Relationships Among the Atelocerata (Labiata) In the previous lecture we concluded that the Phylum Arthropoda is a monophyletic group. This group is supported by a number of synapomorphies that unite the 5 recognized subgroups (trilobites, chelicerates, crustaceans, myriapods and hexapods). We also concluded that the sister group to the Arthropoda is either the Oynchophora or the Tardigrada. However we also discovered that the evolutionary relationships among the 5 subgroups in the Arthropoda are still unresolved. Myriapoda, Crustacea and Hexapoda do seemed united as the Mandibulata, but the placement of the Hexapoda is questionable. Some studies unite the Hexapoda with the Myriapoda, forming the Atelocerata (Labiata in the textbook), while other studies unite the Hexapoda with the Crustacea, forming the Pancrustacea. Still other studies place the Hexapoda within the Crustacea, making the Crustacea, paraphyletic as currently defined. Until more convincing evidence to the contrary we will assume that the Myriapoda and the Hexapoda form a natural (monophyletic) group as does your textbook.
Characters that Unite the Myriapoda and Hexapoda One pair of antennae versus two pair in crustaceans. Metamerically arranged tracheae versus gills in crustaceans. Development of excretory Malpighian tubules from the hind gut versus nephridia in crustaceans. Production of spermatophores versus release of free sperm in crustaceans. Possession of anterior tentorial arms in head capsule versus no tentorium in crustaceans.
Classification and Characteristics of the Myriapoda Diplopoda (millipedes) 21 body segments; body segments fused in pairs each with two pairs of legs (hence the name); lack of second maxillae (dignathous); reproductive organs open at anterior body segment (progoneate); short antennae not branched; 8000 species. Pauropoda (pauropods) 11 or 12 body segments; one pair of legs on each metamere; dignathous; progoneate; short antennae branched; 700 species. Chilopoda (centipedes) 15-170 body segments; at most one pair of legs on each metamere; probably dignathous (possible trignathous); gential opening on last body segment (opistogoneate); long filamentous antennae; 3000 species. Symphyla (symphylans) 14 body segments; at most one pair of legs on each metamere; trignathous; progoneate; antennae long and musculated; 200 species. Phylogenetic relationships among these four classes are not well resolved.
Characters that Unite the Myriapoda and separate them from the Hexapoda Loss of compound eyes. Loss of palps on the first and second maxillae. Presence of organs of Tömösvary at the base each antenna. May be used to detect auditory stimuli. Presence of special repugnatorial glands.
Characters that unite the Hexapoda and separate them from the Myriapoda Fusion of the second maxillae as the labium (also seen in the symphylans). Formation of a distinct thorax composed of three body segments, and a distinct abdomen. Fixation of the maximal number of abdominal segments at 11, plus the telson. Fewer abdominal segments possible through reduction. Loss of all abdominal appendages.
Evolutionary Relationships within the Hexapoda An old theory suggested that the hexapods evolved from the symphylanlike ancestor. If true, this would make the myriapods paraphyletic. This theory is now discredited and certainly would not be true if the myriapods are not the sister group of the hexapods. The Hexapoda is composed of two classes, the Entognatha and the Insecta. Your textbook divides the Entognatha into two classes: the Entognatha (Diplura) and the Parainsecta (Protura and Collembola). Recent evidence indicates that the Entognatha is a monophyletic group that is the sister group of the insects. Some studies suggest that the Diplura is the sister group to the insects because they shared filiform cerci, and characteristics of the sperm. This would make the Entognatha paraphyletic as defined here. Two possible relationships of the Entognatha and Insecta Myriapoda Insecta Diplura Protura Collembola Myriapoda Insecta Diplura Protura Collembola
Classification and Characteristics of the Entognatha Protura 12 segments in the abdomen; minute elongate bodies; functionally tetrapodal, forelegs serve as sensory appendages; antennae absent; 500 species. Collembola no more than 6 segments in the abdomen; minute stout bodies; antennae with few segments; highly modified structures for jumping; 6000 species. Diplura 10 segments in the abdomen; small to large enlongate bodies; antennae long, unbranched; superfically similar to symphylans; 800 species.
Characters that unite the Entognatha and separate them from the Insecta Endognathy (enclosed mouth). Preoral cavity enclosed laterally by pleural folds which grow down from the head and fuse with the labium. Eyes reduced or absent. Reduced Malpighian tubules. Enlongate, sac-like ovarioles.
Major Groups within Class Insecta Class Insecta is divided into two major subgroups, the Apterygotes (=primitively wingless insects) and the Pterygota (=winged insects). The Pterygota appears to be monophyletic, but the Apterygota is not. The Apterygota consists of two orders, the Archaeognatha and the Thysanura. The Pterygota is divided into the Paleoptera (=old wing) and the Neoptera (=new wing). The Paleoptera cannot flex their wings over the abdomen, while the Neoptera can. The Neoptera appears to be a monophyletic group. The Paleoptera is probably not. The Paleoptera consists of two extent orders, the Ephemeroptera and the Odonata. The Neoptera consist of 26 orders, which are divided into one formal group, the Holometabola (11 orders) and two informal groups the Polyneoptera (11 orders) and the Paraneotpera (4 orders).
Archaeognatha (Bristletails) Classification. 2 extant families, one extinct family. 500 species worldwide. Structure. Large compound eyes with continuous medial border. Mandibles are monocondylic (single point of articulation with the head). Apical incisors widely separated from the molar process, which operates with a rolling motion as in crustaceans. Tentorium with very simple structure. Appendages have a ventral articulation with body instead of lateral one as seen in more derived insects. Coxae have small exites (styli), reminiscent of the biramous condition in crustaceans. Abdominal styli present. Specialized jumping structures and musculature. Body covered in scales. Natural history. Active at night, hiding by day under back or in rock crevices. Feed on algae, lichen and vegetable debris. Primitive mandibles used a augers. Moderately good runners and jumpers. Jump by flexing abdomen. Sperm transfer indirect via a spermatophore. Ametabolous (without metamorphosis) development, with immatures closely resembling adults. Molting continues throughout life.
Thysanura (Silverfish) Classification. 5 living families. 370 species worldwide. Structure. Mandibles are dicondylic (two points of articulation with the head capsule). Apical incisors and molar region are not widely separated. Biting is transverse as in more derived insects. Coxae do not have styli. Abdominal styli variably present. Body covered in scales. Natural history. Free-living cryptozoics living under bark and rocks, and in leaflitter. Very common in ant nests. Most species are omnivorous. Some species are subterranean or cavernicolous herbivores. Some species produce cellulase to aid in digestion of cellulose. Sperm transfer is indirect via a spermatophore. Ametabolous development with immatures closely resembling adults. Molting continues throughout life. Lifespan of some species over 4 years.
Conclusions Despite sharing some complex characters, the validity of the group Atelecerata (Myriapoda + Hexapoda) is questionable and several recent studies place the Hexapoda in the Crustacea or as its sister group. The Hexapoda is divided into the Entognatha and the Insecta. The Insecta is clearly monophyletic, but the Entognatha may not be, depending on the placement of the Diplura. The Insecta are divided into the Apterygota (not monophyletic) and the Pterygota (monophyletic), which in turn is divided in the Paleoptera (not monophyletic) and the Neoptera (monophyletic). The Apterygota consist of two orders of small wingless insects, the Archaeognatha (Bristletails) and the Thysanura (Silverfish). These groups show several characters that illustrate the evolutionary transition from the Entognathans to the insects. The Thysanura is the sister goup to the Pterygota.