DIPTERA 



817 



is forced by the movements of the larva into some other part of its- 

 body. Here it continues its growth and development at the expense 

 of the tissues of the mother-larva. Not all of the oocytes complete 

 their development, since usually only from five to seventeen young 

 are produced by a single mother-larva (Hegner) . 



When the tissues of the mother-larva are consimied, the young 

 larvae break forth from the skin of their parent and continue their 

 growth. These larvee may in turn produce another generation of 

 larvae in the same manner. It is believed that this asexual, paedo- 

 genetic reproduction may continue through many generations cover- 

 ing a period of two or three years. Finally a generation of larvae are 

 produced which do not reproduce in this manner, but which when 

 full grown transform first to pupae and then to adults, which repro- 

 duce sexually. According to the observations of Kieffer ('13) the 

 adult females of those species in which paedogenesis occurs produce 

 each only four or five eggs, while other gall-gnats produce a large 

 number of eggs 



Subfamily CECIDOMYIIN^ 



This subfamily includes the larger nimiber of the gall-gnats ; to it 

 belong those species that have attracted attention on account of 

 their economic importance, and others that are well known on ac- 

 count of the conspicuous galls produced by them. Much has been 

 published regarding some of these species ; but unfortunately they are 

 discussed under different generic names by different writers. For 

 this reason the common names 

 will be found more useful when 

 one is attempting to learn what 

 has been published regarding 

 these species. 



The pine-cone willow-gall. 

 — One of the most common 

 and conspicuous of the galls 

 made by gall-gnats is the pine- 

 cone willow-gall (Fig 1036). 

 This often occurs in great 

 abundance on the tips of twigs 

 of the heart-leaved willow, 

 Salix cordata. The gnat that 

 causes the growth of this gall 

 is Rhabdophaga strobiloides. 

 The larva remains in the heart 

 of the gall throughout the 

 summer and winter, changing 

 to a pupa early in the spring. 

 The adult emerges soon after- 

 ward, and lays its eggs in the newly-started buds of the willow. 



The pine-cone willow-gall guest, Cecidomyia albovittdta. — This 

 species breeds in large nimibers between the leaves composing the 



The pine-cone willow-gall. 



