4 WHEELER. [Vol. VIII. 



which they overlap. These differences materially affect the 

 eggs, for many of those thrust in between the closely 

 appressed scales of the spindle-shaped galls are so much flat- 

 tened as to be incapable of developing; on the other hand the 

 eggs deposited in the more spacious interstices of the globular 

 galls are usually in no wise injured. The two forms of gall do 

 not always occur in the same locality and may be the produc- 

 tions of two distinct species of Cccidomyia or of one species 

 on different willows. The Locustids, however, seem to show 

 no preference for the globular galls. 



The galls of Cccidojuyia, being essentially stem-galls, do 

 not drop to the ground in the autumn like the various leaf- 

 galls on the willows, but persist through several seasons. Al- 

 though the insects are not averse to ovipositing in the fresh 

 galls, they nevertheless seem to prefer these blackened and 

 weather-beaten specimens, probably because their scales are 

 more easily forced apart. 



I have called attention to the fact ('90^) that X. ensiferiun 

 departs widely in its habits of oviposition from its congeners, 

 several of which are known to lay their eggs in the pith of easily 

 penetrated twigs, like the species of the allied genus Orchcli- 

 nmm. X. eiisifcnim has evidently found it of great advantage 

 to make use of the galls so abundant in its native haunts. So 

 recent may be the acquisition of this habit, that on further 

 investigation some females may, perhaps, even now be found 

 to have a tendency to oviposit, like Conocephalus cnsiger, be- 

 tween the root-leaves and stems of plants, or even in the plant 

 tissues. It still occasionally happens that the eggs are run 

 through or into the tissues of the gall-scales, and not loosely 

 deposited. The fact that the insects have not yet learned to 

 distinguish the kind of gall best adapted to their purposes, 

 lends some support to the view that it is not so very long 

 since X. ensiferum agreed with its congeners in habits of ovi- 

 position.^ 



1 In the vicinity of Worcester, Mass., I found galls very similar to those formed 

 on the Wisconsin willows. They contained a few slender yellow eggs, smaller 

 than those of X. ensifer%tm. As this species does not occur in New England I 

 conclude that these eggs were probably deposited by the very common X, fascia- 

 turn, De Geer. 



