42 GENERAL BIOLOGY 



As to their distribution upon the plant, galls are solitary, 

 (as in fig. 3 2 a) clustered (as in fig. 32^), or compound (as in 

 fig. 33): they are called compound when they contain 

 separate cavities surrounded by confluent walls. 



The animals that produce galls. With a few unimportant 

 exceptions the animals that cause galls to grow belong to a 

 single family of mites and to five orders of insects, Hemip- 

 tera, Coleoptera, lepidoptera, Diptera and Hymenoptera. 

 The mites are very minute four- or eight- 

 /jhtw legged creatures without distinction of head 

 (~\ and thorax (fig. 316). They live amid the 

 growth of matted hairs that fills the cavity of 

 felted galls. 



Hemipterous gall makers are aphids, 

 psyllids, etc., and they generally live within 

 mantle galls. 



Coleopterous and Lepidopterous gall mak- 

 ers are beetle and moth larvae respectively. 

 They are but a few stray members of large 

 families that are not much addicted as a 

 whole to the gall making habit: but these 

 few make comparatively large closed galls, 

 some of which are sure to be encountered in 

 the following field study. 



Dipterous gall makers mainly are gall 

 gnats (Cecidomyiidae), with a few scattering 

 representatives of other families. Cecidomyiid galls are 

 very common, and of the utmost diversity of structure 

 and appearance. The larvae within them are often very 

 small, but they are distinguishable by the possession on 

 the under side of the first segment behind the head of the 

 so called "breast bone," a flat, brown horny piece that pro- 

 jects forward toward the mouth and is often notched at 

 its tip (fig. 34). 



FIG. 34 Dia- 

 gram of a gall 

 midge larva 

 (family Ceci- 

 domyiida of 

 Diptera). m, 

 the so-called 

 "breastbone;" 

 M, respiratory 

 apertures. 



