22 Journal of the Mitchell Society [December 



would have, in a year or two, a mass of plant lice equal in volume to 

 our earth. This may or may not be true, but if true a large proportion 

 of the descendants must fly away to other worlds than ours because 

 any species is lucky indeed if it ends its fiscal year and balances its 

 books with a definite increase in numbers over the previous year. 



The question of the number of species of insects is, however, an- 

 other matter. We see, in our text books, estimates of the number of 

 species of insects in the world, at anywhere from 250,000 to 500,000 

 and 1,000,000 and recently some one ventured to estimate that there 

 must be at least 10,000,000 kinds of insects in the world, at the pres- 

 ent time. 



Obviously, in trying to generalize about a group of animals with 

 such an enormous number of species, one finds himself handicapped 

 not from lack of material but from its very superabundance. 



General Physiology. 



Typically, an insect is an arthropodous animal with its body di- 

 vided into three parts; head, thorax and abdomen, and with three 

 pairs of legs and usually two pairs of wings. Being an arthropod, an 

 insect carries his skeleton on the outside of his body and as he grows 

 this skin which is hard and chitinous must be cast off and a new skin 

 produced to accommodate the larger sized individual. This skeleton 

 is segmented to provide free movements. Thus various parts have 

 been separated and since these parts are fairly constant they have 

 been much used in taxonomy. But their phylogenetic relations are 

 not always clear and thus a special nomenclature has grown up around 

 each group which has served to discourage all but the most highly 

 specialized of the specialists and has acted as a sort of natural selec- 

 tion, thus cutting down materially the crop of speciahsts, much to the 

 general benefit of the world at large. With these special parts we 

 need not concern ourselves here but the generalized parts will bear 

 closer inspection. The head is largely sensory in function and bears 

 the compound eyes, simple eyes and antennae. These will be dis- 

 cussed more in detail under sense organs. The head also bears the 

 mouth parts which are among the most complicated structures found 

 in the animal kingdom. Primitively, they consist of no less than 

 three paired and three unpaired structures. We cannot inquire into 

 the various morphological variations but the following classification 

 of the mouth parts of insects based largely on functional grounds will 



