1) The main distinguishing characteristic of the Coleoptera is the hardened forewings (elytra) which could be in some cases reduced, six legs and antennae.
2) The "sheath" in the ordinal name refers to the hardened forewings (elytra) that form a protective covering over the membranous hindwings and relatively soft dorsal abdominal wall. "Sheath" just means a covering.
3) All beetles have complete metamorphosis, i.e. egg-larva (several instars)-pupa- adult. It is the combination of an insect with complete metamorphosis and the characteristic hardened forewings (elytra) that characterize insects that are called Coleoptera.
4) Besides the usual "positive" role of beetles, like most other insects as food for amphibians, reptiles, fish, birds and mammals, there are some beetlesm that play very important roles in the environment, i.e. many are pollinators (cantharids, scarabs, byturids, etc.);
dung beetles (scarabs, geotrupids,etc.) feed on and reproduce in dung of herbivores, thereby removing millions of tons of dung that would accumulate and destroy valuable pastureland and natural areas;
burying beetles (silphids) bury animal carcasses that are used as food by the adults and their growing offspring, thereby ridding the landscape of carcasses that would otherwise contaminate and foul the environment;
various nest-dwellers (histerids, trogids, staphylinids, etc.) and fur ectoparasites (leiodids, leptinines, etc.) rid their hosts (birds, mammals) of parasitic insects, such as fleas, bed bugs, lice;
some beetles are very effective as biocontrol agents feeding on plant-feeding insects, e.g. lady beetles (coccinellids) feed on aphids and scale insects - the Vedalia lady beetle in California reduced the scourge of the cottony cushion scale (Icerya purchasi), carabid beetles, such as the "caterpillar-hunters" (Calosoma, Carabus spp.) are often helpful in reducing populations of harmful caterpillars, such as gypsy moth and budworm caterpillars.
fireflie larvae (glowworms) eat slugs and snails which do damage to crops, mainly tomatoes and lettuce.
These are just a few examples of the beneficial aspects of beetles, not to mention the delight one can find in their aesthetic beauty and remarkable diversity!
If you are interested in reading a nice introduction to the beetles, other than what can be found in the Peterson Field Guide to the Beetles by Richard White, or in an elementary textbook of entomology, look for a copy of the following book in a library, or on the used book market:
Branson, Wilfred S. (1963) - Beetles. Harcourt, Brace & World, Inc., New York. 160 pages.
Good luck with your beetle studies - a world of wonder awaits you!
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Last modified on Tuesday, 12 November 2019