I have spent a considerable amount of my time over the years studying the largest known class of living things, insects. My interests have ranged from dragonflies to beetles and from butterflies to wasps, bees and ants. Although my major research was in an allied arthropod class, arachnids (primarily spiders,) I earned my living primarily through my work with six-legged critters. I still remember the red-letter days of summer in Arizona when, as a teen-ager, I would cross an unsprayed alfalfa field swarming with butterflies, bees, true bugs, wasps, and various other insects. This was, to my way of thinking, a demonstration of the gloriously vibrant nature of life on earth. Such experiences set me on the road I eventually traveled to being a professional biologist.
Libellula saturata, a bright orange dragonfly.
Insects seem foreign, even otherworldly, to many if not most people. However one could argue that we, ourselves, are among the strangest of potential aliens of all. Life on our planet ranges from super tiny viruses (which some would argue are not living in a "normal" sense,) through amoebae, bacteria, archaea, fungi, vascular plants, sponges, cnidarians, worms of several phyla, mollusks, arthropods (including insects, spiders, crustaceans, and others) echinoderms, and vertebrates, including us. If we were honest we would see these varied life forms as really strange products of a very strange history on the most remarkable planet in our system. Insects are just one manifestation of the basic weirdness of the planet and of the universe. That they seem alien to us is an artifact of our rather provincial views. Perhaps they should be considered as more typical of life on earth than the vertebrates, especially since they outnumber these both in individuals and species.
Insects never reach the size of most vertebrates (although some vertebrates are the size of some insects) and they in fact never get beyond about 30 cm in length (some dragonfly relatives in the Carboniferous were longer, but as in our modern-day longest insects they were very thin) and thick-bodied insects are rarely over 10 cm. Why is this? I posed this question at a local Christian school when I got a request to give a talk there (I do not discriminate) and got back the answer "Because God made it that way." This was not the answer I wanted, needless to say. What I was looking for was a physical reason that insects cannot grow above a certain size. There are several reasons, one of which being that they have an exoskeleton (external skeleton) that works well at a small scale, but becomes a burdensome structure above that size. Imagine a six meter cockroach (think,"Men in Black.") In order to reach full size the giant roach would have to molt its exoskeleton several times. With each molt its hardening period would lengthen and its muscles would become less efficient. Above a certain size it would simply not be able to move! Another problem is the respiratory system, which depends on a rather inefficient network of little tubes called tracheae. There is no respiratory pigment like hemoglobin in the blood to carry oxygen to the cells as in vertebrates. Tracheae work fine at the normal insect scale, but would not function well at larger sizes.
A solitary wasp nectaring (family Sphecidae).
Insects can be defined as organisms with three major body parts and usually six legs as adults. They also often have wings as adults and generally have hardened exoskeletons (as do other arthropods.) They share the six leg characteristic with the proturans, collembolans, and diplurans, rather obscure wingless creatures that are considered as belonging to a primitive sister group to the insects. Unlike arachnids, which have a copper-based oxygen-carrying pigment in their blood, insects depend totally on atmospheric pressure and the lateral valves called spiracles to pass oxygen to their cells.
Insects are marvels of microminiaturization, with some very functional insects being less than 1 mm in length. They live in just about any environment, except the poles and the oceans, where their relatives, the crustaceans (crabs, shrimps, etc., now broken up into several classes) dominate. The ants (order Hymenoptera, family Formicidae) alone could be said to be the dominate multicellular organisms on the face of the earth by themselves. Their sheer number of individuals causes them to actually have a biomass about as large as the entire human population, even though none is much larger than 25 mm long. This is not a species by species comparison, however, as there are many species of ants, but only one human species. I have had a few experiences with several very interesting ants, from the trap-jawed ants in the genus Odontomachus to the remarkable leaf-cutting ants in the genera Atta and Acromyrmex, and the ferocious army ants in the genera Neivamyrmex and Eciton. In the southern Animas Mountains of New Mexico I found the "slave-making" Amazon ants (Polyergus) with their captured workers in the genus Formica (all Amazon ants, except for the reproductives, are soldiers and cannot even feed themselves.) I should note that all worker and soldier ants are neutered females, unlike termites, where both sexes take part in the work and defense of the colony.
A white-lined sphinx moth (family Sphingidae) nectaring at phlox.
Ants, bees and wasps (order Hymenoptera) are not the only interesting groups of the six-legged class. Just about all insects (and other arthropods) are as interesting. Caterpillars with venomous spines that metamorphose into the beautiful Io moth or the dull hag moth, grasshoppers that turn into huge swarms of locusts in Africa, mosquitoes that transmit some of our most dangerous diseases, beetles that cooperate in burying dung or dead animals to rear their young, fierce metallic tiger beetles chasing their prey across a mudflat, equally fierce robber flies that look like bumble bees and take their prey on the wing, helicopter damselflies that pluck spiders from their webs, and on and on, make up this highly diverse class. Insects are endlessly fascinating and are useful in understanding environmental systems, genetics (think laboratory fruit flies), for pollination (bees, butterflies, wasps, flies and some beetles come to mind), and for controlling other arthropods.
Two metallic wood-boring beetles (family Buprestidae, genus Acmaeodera) on composite bloom.
Complete metamorphosis (found in the larger number of insects) is a remarkable process in its own right. The mysteries of this transformation have slowly been revealed, but there is still a lot we don't know. How larvae very different from the adult stage evolved is still hotly debated and has been muddied in recent years by a very strange idea that I discussed in another diary a while back (See: http://www.dailykos.com/....) The HOX, or homeobox, genes are now known to be the genes that determine body shape. These same gene complexes are found in most, if not all, organisms and point to a single origin of all living things. Still, we know how the development from caterpillar to butterfly works as a process, but not yet how it evolved or why. Also why the huge diversity of forms? This question was once answered by the concept of a creator who fashioned all with a flick of the hand or a word. However the idea of an "ethical" creator who makes ichneumon wasps, which act like the Alien in their early stages, certainly gives me the creeps, as it did Darwin. As a product of a mindless process like evolution, such a life strategy becomes remarkable. We are still left with the mystery, which we, in the ultimate, may or may not solve before we join the passenger pigeon in extinction (perhaps sooner than we think!) My suspicion, based on my early training in the scientific method, is that ultimate questions are beyond our ken because they cannot be tested. We are more or less limited to proximate questions, some of which are very important and for which science is a powerful tool.
Finally, I would like to encourage people to observe our smaller organisms. They will be, in my opinion, well rewarded, and if they really get interested they can actually add to our knowledge of these important creatures. Just by casual insect observation and photography I have added several new species records of dragonflies for my county and one new state record for a Mexican butterfly species. Studies on the behavior of insects and other arthropods are needed, but not often undertaken by professionals anymore because they have to work on projects for which they can get grant money. A well-informed amateur can also contribute to taxonomy, although DNA studies are now more prevalent for such. Still these do require money and non-economic species are thus neglected. To repeat an old truism we just don't always know what may turn out to be important to the environment and our understanding of it. Insects make up a vital part of that environment.
As to references for information on insects and their relatives there are so many books and Internet sites that it is hard to pick only a few. Several good insect guides have appeared in the last ten years or so, and for behavior it is useful and inspiring to consult Fabre's antique volumes on "The life of the Grasshopper," "The Hunting Wasp," "The Sacred Scarab," etc. These are out of date and Fabre misunderstood some of the things he observed, but they do capture the fascination of the subject. Literature can certainly help, but nothing can take the place of observation.