1920] Metcalf: Diptera 185 



A host of species are found in decaying vegetable or animal 

 matter in every possible stage of disintegration (Psychodidae, 

 Muscidag, Drosophilidas, Bibionidae, Mydaidae, Asilidae, Heter- 

 oneuridae, etc.) We have all marveled at the adaptations 

 which enable an ephidrid fly to live as a larva in pools of crude 

 petroleum, a substance highly toxic to almost all insect life.* 



Compared with the Coleoptera, Lepidoptera, Hemiptera, 

 and Orthoptera, Dipterous larvae are rarely phytophagous on 

 living plants. There are comparatively few crop pests. Yet 

 many are leaf- miners, a good many feed in fruits, seeds, etc., a 

 number are borers in the cambium and other parts of the 

 trunks and stems and some feed exposed on the surface. The 

 Cecidomyidae or Itonididae are almost exclusively phytophagous 

 and exhibit the most remarkable and intricate and inexplicable 

 adaptations to the abnormal plant structures they occasion 

 and to securing food. 



The larvae of many Tabanidae, Syrphidae and Asilidae are 

 predaceous on insects and other small animals. Certain leptid 

 larvae are said to construct conical pitfalls for ensnaring prey, 

 after the manner of ant-lions. 



Many species of the Bombyliidae, Phoridae, Pipunculidae, 

 Conopidae, Cordyluridae, Sarcophagidas and Tachinidae are 

 parasitic on other insects. A few are parasitic on warm- 

 blooded animals, the Hippoboscidse as ectoparasites, the 

 Oestridae and in part the Muscidas, Sarcophagidae and Syrphidae 

 as endoparasites. Certain species of the Muscidae are unique 

 in having larvae which are intermittent blood-suckers on 

 mammals and birds. 



THE PUPA. 



One of the most noteworthy features of this order is the 

 clever manner in which the last larval exuvium is utilized as 

 a protection for the pupa stage. Cases aside from the Diptera 

 may be cited where the larva retains its exuvia, more or less 

 mixed with excrement, as a covering during the quiescent 

 transformation period, but I know of none which approach 

 the diptera in the perfection of this habit. 



In the suborder Cyclorrhapha entirely and in many of the 

 Orthorrhapha the last molt is not cast at all, but becomes 



*See Crawford, D. L., The Petroleum Fly in California. In Pomona Col. 

 Jour. Ent. IV, 2, May, 1912, pp. 687-697. 



