186 Annals Entomological Society of America [Vol. XIII, 



inflated and indurated around the pupal membrane to form a 

 complete, waterproof and resistant box within which the pupa 

 stage is secure. A pair of thoracic spiracles is projected through 

 this puparium for the respiration of the pupa. Quite as unique 

 is the adaptation by which, in several dozen families, an 

 inflatable sac, the ptilinum, is projected through the frons to 

 dislodge the cap from the puparium and permit egress of the 

 fly. So far as it is possible to formulate rules, it is the rule 

 for the pupa stage to be passed in or near the larval habitat. 

 In the case of aquatic or semi-aquatic larvas the pupa is found 

 in drier situations nearby. It is adapted to float on the water 

 in the Psychodidae, Chironomidae, Stratiomyidse and Ephy- 

 dridae. It may even survive successfully on the bottom of 

 pools (Chironomidae), or under water in streams or lakes, as 

 in certain Simuliidae, Blepharoceridae and Ephydridae. 



In the case of the parasitic species the pupa may remain 

 in the host (Conopidae) or leave the host and seek protection 

 on the ground (Tachinidse and Oestridae). In the ectoparasitic 

 Hippoboscidae this stage may be glued to the hairs or feathers 

 of the host or lie on the ground. 



As to the duration in the pupa stage we note the same 

 variation as for the other stages. As a rule this stage is a 

 rather short one, exceptionally occupying only a few hours, and 

 commonly from a few days to a few weeks. But it is very often 

 utilized as a hibernating stage, and in remarkable cases a 

 second winter may be passed in this condition (Trypetidas) . 

 This tendency of part of the brood to be delayed over an entire 

 additional year as exhibited by the eggs of certain mosquitoes 

 and the puparia of some fruit flies is a most inexplicable adapta- 

 tion for the preservation of the species. 



The exact length of the pupa stage is seldom recorded 

 because of the fact that a prepupal stage of most irregular 

 length may occur after the puparium becomes indurated before 

 the change from larva to pupa takes place. 



In the matter of activity of the pupa certain families of the 

 Diptera are remarkable. The mosquito pupa swims about 

 actively and avoids enemies in a manner most remarkable for 

 a stage traditionally quiescent. 



