WEEK 2: INSECT MACROEVOLUTION Forty million years ago some insects were trapped in tree resin and preserved in what became amber. These trapped insects look almost exactly the same as insects around us today. This means that most of present-day insect diversity was already well-establisheinsects, we have to go 10 times as far back 40 million years ago! To see truly primitive Several key innovations punctuated insect evolutionn and contributed to their great success. I ll go through these in order. Insects are arthropods (Phylum Arthropoda) and they share many features with the Annelids (Phylum Annelida) and the Onchophoranss (Phylum Onychophora). These three phyla almost certainly shared a common ancestor a very very long time ago (~ ~500 million years ago). (parapodia). The Annelids (segmented worms) are characterized by soft bodies which are elongate, bilaterally symmetrical, and whichh are segmented. Their bodies consist of a row of repeating segments called metameres. In many annelids, each segment has a bi-lateral pair of appendages The Onychophorans are a small group of terrestrial animals found in humid environments of the tropics and subtropics. They share features with both annelids and arthropods. They have elongate, worm-like bodies with lobe-likee legs. The segments are especially clear in embryos.
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 ENY3005/5006 Principles of Entomology, University of Florida
Subdivision of the body into head, thorax, and abdomen facilitated invasion of new habitats because animals became MUCH more specialized. The head region became more specialized for feeding, tasting, and seeing. The thorax became specialized for locomotion, and the abdomen for reproduction. The earliest Hexapod known is a fossil springtail (Collembolan) ~380 million years old. These very primitive hexapods also include the Proturans and Diplurans. Some very primitive true insects include the Archaeognatha and the Zygentoma. All of these primitive insects were terrestrial and wingless, called apterygotes. By the end of the Devonian (~315 MYA) the first winged insectss had appeared. Flight was a huge advantage. These pterygote insects were extremely successful. Just as invasion of the land opened up whole new habitats for colonization, so, too, flight opened new worlds. These first flying insectss flew in forests of giant ferns, club mosses, and horsetails. Colonization of the air led to the first great radiation of the insects, and by the end of the Carboniferous, wholee new orders of insectss had appeared. These included many orders now extinct, like the Paleodictyoptera which sometimes reached wingspans of over ½ a meter! Many orders went extinct during the Permian. Two orders thatt arose in the Carboniferous that are still represented today are the Ephemeroptera and the Odonata. Another major advance that occurred in the Carboniferous was the evolution of a wing flexion mechanism. This allowed insects to fold their wings up and place them over
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 ENY3005/5006 Principles of Entomology, University of Florida
WEEK 2: INSECT MACROEVOLUTION A member of the order Archaeognatha Study Questions and Objectives List the four major innovations in insect evolution. Why were they important? Explain what led to the two great radiations of insects Clarify the difference between being primitively apterygote versus secondarily apterygote. From last week s reading and the class exercise this week: Put these groupings in order: Class, Phylum, Order, Genus, Species, Family. Be able to do the same for a given species of insect. What are four other groups of arthropods, other than insects? What are two groups of non-insect hexapods? What was the common ancestor to the three phyla of Onychophora, Annelida, and Arthropoda? Which of the pictures below is an Onychophoran, which is an Annelid, and which is an Arthropod? Study Terms Phylogeny Ametabola, Hemimetabola, Holometabola Apterygote, Endopterygote, Exopterygotee Neoptera, Paleoptera,Homoplasy, Homology, Convergent Evolution