Dr. Miller 

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006 Principles

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Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida 

The Arthropods (joint-footed animals) are like annelids in that arthropod bodies are segmented. However, their segments are grouped into functional units called TAGMATA. Arthopods are found everywhere, and one key feature contributing to their success was a relatively impermeable exoskeleton.

The common, ancestor to these three PHYLA was an aquatic animal, a marine worm. Each of these three phyla eventually succeeded in colonizing terrestrial habitats, but they probably did so independently and in different ways. In the case of ARTHOPODS an impermeable exoskeleton played a crucial role. The first step was the evolution of a tracheal system. Prior to this, all gas exchange had to occur through the body wall… this meant that the outside layer of the animal could not be impermeable (if it was, no gas exchange could occur…). If animals with permeable skin left the water, they would dessicate and die. Evolution of the tracheal system did two things: 1) it made gas exchange more efficient (probably the reason tracheal systems evolved in the first place). 2) it localized gas exchange to gill pads or to spiracles. Once gas exchange was LOCALIZED, these animals were free to evolve more protective and therefore impermeable cuticles over the rest of their body surface. This probably resulted from selection for protective armor, but once the Arthropod exoskeleton was protective and impermeable, this paved the way for at least some arthropods to invade land.

HEXAPOD EVOLUTION: Hexapods evolved from a terrestrial, segmented, worm-like ancestor and from a body plan of iterative repeats. From this arose a body form with segments fused into three distinct, functionally specialized regions. Anterior segments became modified with addition of eyes and by a pair of appendages serving as antennae. At some point, the bi-lateral appendages became segmented, permitting greater degrees of task-specificity and morphological specialization.

Dr. Miller 

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006 Principles

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Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida 

their body when not in use. These Neopterans could now inhabit all kinds of rougher environments that would previously have shredded or mutilated their wings. Because they could fol d their wings, they could protect them, burrow into soil, crawl under logs

and rocks, etc. Neopterans were very successful. Today they are represented by 90% of the orders and 97% of the world’s species. Some of these early Neopteran orders included the Blattodea and the Orthoptera.

The origin of wings, and later, the flexion/hinge mechanism, facilitated a great radiation of insects. These insects dominated during most of the Carboniferous period. But, the Permian witnessed a massive decline in the ferns and mosses, and with this came extinction of lots of these early orders. It wasn’t until ~150 million years later (the Cretaceous) that several more key innovations launched the second major evolutionary radiation of insects.

The last huge step was the appearance of complete metamorphosis (Holometaboly). In all prior species, wings had to be developed gradually as flaps on the outside of the nymphs. This meant that 1) nymphs had to physically resemble the adults, and 2) nymphs were “fragile”, tough environments would damage developing wing buds. Holometaboly permitted adult traits (wings, legs, eyes, genitalia) to be produced inside the larval body where they are protected. ALSO, this feed larvae to colonize new, rougher habitats, and to become morphologically specialized for those habitats. The evolution of holometaboly launched the second major radiation of insects.

These endopterygote (wings inside) orders include the Diptera, Lepidoptera, Hymenoptera, and the Coleoptera. Endopterygote larvae became morphologically, behaviorally, and physiologically specialized for growth, while adults of the same species became specialized for mating, reproduction, and dispersal.

OF all the neopterous insects, the endopterygote orders are the most derived, and, by far, the most successful (in terms of the number of species, number of individuals, etc.)

A second major even occurred at about the same time as the evolution of the endopterygotes. The first flowering plants appeared (Angiosperms). Winged insects, as herbivores and pollinators, diverged in tandem with the angiosperms (this was coevolution). So the Cretaceous period witnessed a huge radiation of insects, and it was this

second (last) great radiation that produced all of the extant orders and most of the families of insects that we observe today. And, these are the insects we find so perfectly preserved in Amber.

Dr. Miller 

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006 Principles

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