CLASSIFICATION OF INSECTS. 9 



external appearance represent numerous independent lines of 

 development. 



In almost any group of insects there is easily seen the great ease 

 with which adaptations inform appear, most commonly in response 

 to the requirements of alimentation and respiration. Among these 

 might be mentioned many grotesque modifications of the trophi 

 of entomophagous parasites, the posterior closure of the alimen- 

 tary tract, the great flattening of the body in many subcorticolous 

 species, and the development of respiratory tubes, gills, etc., in 

 aquatic forms. In details, the active, free-living types of larvae 

 naturally furnish the most striking and clear-cut characters, 

 while those of sessile, or at least inactive habits show such a 

 reduction or complexity in structrue, that they tend to converge 

 toward a common type. There are notable exceptions to this 

 last statement, however, as for example among the maggots of 

 the higher Diptera to \vhich I shall return later. 



During recent years the larvae of mosquitoes and those of some 

 other Nematocerous Diptera, have been examined with perhaps 

 greater care and thoroughness than those of any other insects. 

 Were it not for this, no one should think of considering the highly 

 specialized larvae of this group before dealing with those of the 

 more generalized insects. Mosquitoes are well-known as re- 

 markably adaptive insects in many respects. The adult mos- 

 quito (at least the female) is phlebotomic and depends upon the 

 higher vertebrates for its food. It further shows a complex 

 relationship to its vertebrate food-reservoirs through the medium 

 of certain Protozoan parasites. The malarial parasites of man 

 undergo their definitive life cycle in the mosquito and the same 

 is true of various other similar Protozoa parasitic in the hosts of 

 other mosquitoes. As is well known to every one, the larvae of 

 mosquitoes are very active aquatic creatures of very peculiar, 

 although quite uniform conformation. It is perhaps not so well 

 known that many of the species are restricted quite closely to 

 certain places, e. g., ponds, pools, rain-barrels, brackish water, 

 permanent water, temporary water, etc., in such a way as to 

 show that they thrive ordinarily only under quite specific con- 

 ditions. Nearly all are surface breathers and are equipped with 

 a breathing tube into which the main lateral tracheal trunks 



