OF WASHINGTON. 419 



parasitic habit by entrance through the mouth. These modifica 

 tions in the CEstridae, as compared with the Muscidae, have doubt 

 less taken place consectaneously with the development of the 

 mammalian type. 



The frequency with which Dipterous larvae of various families 

 succeed in entering the air-passages or the stomach, even of man, 

 is attested by the numerous records of the spitting up or excreting 

 of such larva. A most remarkable fact is that some of the species 

 appear not only to live but to be able to develop for some time in 

 the intestinal heat of the stomach, and to be passed without 

 injury. In the case of some of the flat-flies (Homalomyia spp.), 

 which, living normally in putrescent animal or vegetable matter, 

 are furnished with lateral gills, this power to live in the stomach 

 of man is not so remarkable ; but all these cases of partial and ex 

 ceptional parasitic life under these abnormal conditions strikingly 

 illustrate the manner in which truly parasitic habits may be ac 

 quired. 



The Hippoboscida3, again, show very clearly their connection 

 with free forms, inasmuch as most species are still well supplied 

 with wings. Some possess wings when first reaching the imago 

 state and lose them after establishing themselves on a suitable 

 host, while the greatest differentiation from the normal Dipterous 

 characteristics is found in the species which never acquire wings. 

 They are doubtless an offshoot from some of the more generalized 

 Muscidaa, in which family we find, indeed, among some of the 

 commoner forms, the habit of retaining the larva after this last 

 hatches. Abnormal, therefore, as is the development of the Hip- 

 poboscids, there is every reason to believe that it is but a modifi 

 cation of the normal habit in the Muscidas a modification that 

 has undoubtedly been aided by the fact that their chief hosts, 

 namely, the owls, are a peculiar group of birds, essentially noc 

 turnal, and fond of congregating in caverns and dark places. 



In the Nycteribiids, or bat-flies, the differentiation or degrada 

 tion has been carried still further, in accordance with their still 

 more perfect parasitic life. This family is mostly confined to 

 bats, the peculiarities of which, in being essentially nocturnal and 

 in clustering in large numbers in caves and dark places during the 

 day, would all help to produce specialization and differentiation 

 of the peculiar parasites which infest them. 



