188 Annals Entomological Society of America^ [Vol. XIII, 



among the Hippoboscidse, of individuals puncturing flies of 

 their own kind and sucking the blood which the latter are 

 drawing from the warm-blooded host. It is said that chains of 

 these flies three or four individuals long may be found, taking 

 the blood successively from their more fortunately (or unfor- 

 tunately!) situated associates. The Hippoboscidae are unique 

 among permanent ectoparasites in possessing wings. 



Blood appears to be especially effective on the development 

 of the eggs. For example in certain Muscidae several feedings 

 of blood are necessary for the development of each batch of 

 eggs. It is said that Aedes calopus, after feeding the first time, 

 lays eggs and then becomes nocturnal in habit. 



The mouth-parts of the adults, like those of many of the 

 larvae, are extremely specialized away from the ancestral con- 

 dition. Within the family, however, they are, superficially 

 at least, rather homogeneous; the species adapted for feeding 

 in the several ways are hardly as diverse in structure as one 

 would expect. The mouth-parts of the blood-sucking Muscidae 

 are probably unique among piercing insects in having the labium 

 specialized as the cutting apparatus and adapted to enter the 

 wound. The proboscis is rarely extremely elongated, an Indian 

 tabanid having a beak an inch and a half long and three times 

 the body length ; this is probably used in probing flowers. 



The number of generations a year varies in the Diptera from 

 one, as in Oestridae, certain Tipulidae, Syrphidae, etc., to two 

 or three, which would seem to be the most common condition, 

 (Tabanidae, Cecidomyidae, Sepsidae, Syrphidae, Trypetidae, Anth- 

 omyidae, etc.), to 7 or 8 in certain Muscidae, 8 or 10 in some 

 Trypetidae, Simuliidae and Culicidae, and as many as 20 in 

 Drosophilidae. 



The number is determined (1) by a certain inherent minimum 

 time for each stage in each species and (2) by environmental 

 conditions of heat, moisture, food, etc. 



It may be said that the dipterous life cycle is noteworthy 

 for its shortness in certain species, rather than for any remarkable 

 prolongation or slowness of development. The "shortest com- 

 plete generation is about 10 days for the house fly, fruit fly, etc. 

 while a duration of more than one year is very unusual. 



The method of passing the winter is an interesting problem, 

 but unfortunately one on which we have little information. 



