Spring and now summer have brought with them the usual amount of diversions (family, work, field-trips etc.) as well as another (near) family tragedy. I have not forgotten Ento. 101; indeed I have missed my time sequestered in the Nature Study, perusing books and websites, learning more about the fascinating world of bugs. Now that the year is turning, and bugs are less likely to be encountered, I will continue…
Tagma (upper) and wing of conehead (Orthoptera)
Previously I labeled the insect thorax, which can be described as the anatomical base for locomotion because it carries both the wings and the legs.
The wings arise in pairs from the dorsolateral areas of the mesothorax and metathorax, while the legs are attached to the ventrolateral areas of each of the three thoracic segments. Wings are the unique feature that seperates insects from other arthropods, making insects the only invertebrates capable of flight.
An Apterygot Silverfish
However, not all insects have wings, not all wings are neccessarily involved in flight, and not all winged insects can fly. Insects are in the class Hexapoda, which are traditionally¹ subdivided into two sub-classes: Apterygota, which are primitive wingless insects and the Pterygota, which are winged and secondarily wingless insects. The wingless Apterygota orders are the bristletails (Thysanura), two-pronged bristletails (Diplura), the proturans (Protura) and the springtails (Collembola). The secondarily wingless (i.e those that have descended from winged ancestors) Pterygota orders are the fleas (Siphonoptera) and lice (Phthiraptera). Functional wings are usually found only in adult insects and the last instar stage (the subimago) of mayflies (Ephemeroptera).
Besides flight, wings can perform a variety of functions. Forewings may be modified as protective covers, either by hardening, as in beetle elytra; or as leathery tagma, as in grasshoppers (Orthoptera), cockroaches (Blattoadea) and earwigs (Dermaptera). Hemelytra are wings that are half-leathery but with membranous tips. Flies appear to have only one pair of wings (hence Order Diptera: from the Greek di = two, and ptera = wings), however, the hindwings on the metathorax are reduced to structures called halteres. Butterflies and moths (Order Lepidoptera, from the Greek ‘scale-wing’) have wings that are covered with scales. The Order Trichoptera (Caddisflies), related to the Lepidoptera, have hairs on the wings rather than scales. Thrips (Order Thysanoptera, from the Greek ‘fringe-wing’) have wings with a hair-like fringe, although some are secondarily wingless. Wings can serve as thermoregulators, camouflage, visual warning signs, sexual attractants and in some insects they provide a resonating surface for alarm, territorial or mating calls.
How did wings originate? We have no firm evidence for wing origins. The classic theory is that wings developed from paranotal lobes–fixed extensions of the thorax that were used for gliding and that eventually became articulated. Others believe that the origins lie in adaptations from outgrowths (exites) from a basal segment of the legs, which have now vanished. Others point to the gill plates of mayfly nymphs as a possible indication of an originating structure (a homologue). The fossil record indicates that wings developed before the late Carboniferous period, over 360 million years ago. The dragonflies and damselflies (Odonata) and the Mayflies (Ephemeroptera) are considered the most basal flying insects, grouped together as Palaeoptera(‘old wing’), who have wings that cannot be folded back along the abdomen. All other insects are grouped in the Neoptera (‘new wing’), because they have developed the ability to flex the wings by means of unique muscular arrangements which allow them to fold back over the abdomen. Due to the commonality of wing placement (on the meso- and metathorax), the identical structure (membranous cuticle supported by veins), the consistent patterns of wing venation across all orders, and the common wing musculature (which varies only in the Odonata); wings are considered to have evolved only once from a single ancestor.
Next: Ento 101–Wing Structure and Venation
Some web articles on wings:
♦ Gallery of Insect Wings
♦ Insect Flight Muscle
♦ The hidden beauty of insect wings
♦ The Origin of Insect Wings
As usual, comments and corrections are welcome!
¹Current ideas on phylogenetic relationships will be discussed later in the chapter: Systematics and Classification
Norman F. Johnson and Charles A. Triplehorn, 2004. Borror and DeLong’s Introduction to the Study of Insects (7th edition, pp 12-14) Brooks Cole.
Grimaldi, David and M.S. Engel, Evolution of the Insects, (pp. 128-130) Cambridge University 2005.
Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, (pp.1186-1192) Elsevier 2003.