No. 64] DIPTEHA OF CONNECTICUT 5 



water genus of mosquito, may remain dormant on the ground for 

 months or even years. 



The hirval period of many flies is very short under favorable 

 conditions, but that of some species is quite long. Temperature has 

 an important effect, as larvae grow rapidly in warm weather, so one 

 generation may have a longer developmental period than another. 

 The larvae of the little vinegar fly, Drosophila. complete their de- 

 velopment in 3 or i days in the laboratory. ]SIosquito larvae and 

 housefly larvae develop in a very few days in the summer. The 

 develo])ment of tachinids is generally correlated with the life cycle 

 of the host, and the larval period may be days or months in length. 

 Bot-fly and warble-fly maggots have a developmental period months 

 in length. Some species of black-flies, crane-flies, and horse-flies 

 hibernate as larvae. 



The pupal period also shows much variation among species, and 

 temperature again mav be the determining factor in the variability 

 within a species. Pupation may occur in the fall, and adult emer- 

 gence in the spring. AVhen hibernation occurs in the larval stage, 

 the pupal period in the spring is relatively short. Species with many 

 generations a year must obviously have a short pupal period during 

 the summer. 



The length of the life cycle, from egg to adult, is determined 

 by both inherent and environmental factors, as is indicated by the 

 remarks on the duration of the different stages. A warm summer 

 may produce more generations of multivoltine species than one that 

 is cool, and continued warm weather well into the fall may increase 

 the number of generations. The occurrence of frequent rains during 

 the summer may result in an increase in the number of generations 

 of mosquitoes as compared with a driej' season. Species that are in- 

 herently univoltine are not so markedly affected by temporary fluctua- 

 tions in environmental factors during the summer, and the phenom- 

 enon of a single generation a year remains unaltered. 



A considerable difference in size among individuals of the same 

 species of fly is sometimes noticed. This is usually due to two factors, 

 a sexual difference and a variation in food supply. The females of 

 some species are larger than the males. If the food supply is not 

 sufficient for normal growth, the larvae may mature, but the adults, 

 although perfect in form, will be small. 



Most species of flies exhibit a fairly obvious diff'erence between 

 sexes. This is found in the abdominal structures if nowhere else. 

 The tip of the adbomen of the male is strikingly unlike that of the 

 female, due to the development of external sexual organs. The male 

 genitalia may be held partly withdrawn in the tip of the abdomen 

 or they may be conspicuously exposed, as in crane-flies and many 

 dolichopodids. In addition to this difference in genitalia, the anten- 

 nae, eyes, and even wings of the sexes may show marked differences. 

 The antennae of male mosquitoes and chironomids, for example, are 

 much more hairy than those of females. The eyes of males of many 

 species are closer together at the top of the head than are those of the 



