1920] Metcalf: Diptera 183 



All the eggs may be matured and extruded at one period, 

 after which the female usually dies rather promptly. Among 

 Leptidas and Simuliidce several females may contribute to the 

 same egg-mass and their dead bodies are found together. Or 

 the eggs may mature in definite batches at successive intervals. 

 Or they may mature gradually and continuously and be laid 

 ove'r a period of several days as matured. 



It is puzzling to explain certain indirect methods by which 

 the larv£e are obliged to reach their proper feeding grounds. 

 The human bot fly, Dermatobia cyaniventris is said to lay eggs 

 on the bodies of a mosquito which serves as the porter for trans- 

 ference to a warm-blooded host, the contact with which causes 

 prompt hatching. The well-known cases of the common 

 Oestridae in which the eggs are laid on a part of the body of 

 the host remote from that occupied by the larva would seem 

 to be a highly inefficient habit. The eggs of certain Syrphidae 

 may be laid on plants in anticipation of the usual subsequent 

 infestation by aphids on which the young depend. 



The duration in the egg stage is extremely varied, ranging 

 from as few as four hours in the case of certain Muscidas to 

 commonly two or three days, or as many weeks. There are 

 cases in which the winter is passed in the egg stage; or, most 

 remarkable, where an entire year and a second winter of 

 dormancy may be followed by successful hatching, as in certain 

 mosquitoes. 



As a rule, a considerable number of eggs are developed, but 

 this -varies even in a single family like the Tipulidas from 45 to 

 2,000. Other species which are reputed to lay large numbers of 

 eggs are certain Simuliidae, Cyrtidae, Trypetidae, Muscidae, and 

 Tabanidae. From the data I have noted it would seem that the 

 average number of eggs per female is between 100 and 200. 



THE LARVA. 



The larval stage of the Diptera shows extreme specialization, 

 being further removed from the typical arthropod condition 

 than any other order. In the majority of the famiHes this stage 

 is a somewhat degenerated one and in many of them extremely 

 so. True thoracic legs are always wanting and only rarely are 

 pro-leg-like structures present. A definite head with well 

 developed mouth parts and sensory organs is wanting in all the 

 higher families. The condition of the spiracles also shows 



