9 6 



THE YOUNG NATURALIST. 



[May, 



mation is deliberate; of the three hooks, only two being fitted to 

 perform the functions for which they exist, and, in all probability, but 

 one properly so. Aulax hieracii, another gall fly (Fig. 5.), has three 

 fairly well developed hooks, while Synergus incrassatus (Fig. 6.), a lodger 



Fig. 7. 



F'g- 5. Fig, 4, Fig. 6. 



in galls, possess a similar number, but all perfect. In Nematus galli- 

 cola, a sawfly (Fig. 7), is an example of irregular hooks, they being 

 nine in number, but scattered all along the costal nervures. In all 

 instances, the free ends of the hooks are recurved, to prevent lacera- 

 tion of the folded membrane of the upper wing, provided for their 

 reception. 



(To be continued). 



On the Periodic Abundance of 

 certain Lepidoptera. 



By W. E. SHARP. 



I have been greatly interested in Mr. Corbett's article in the 

 February number of this magazine on the causes which lead to the 

 erratic appearance of certain lepidoptera. The subject has a kind of 

 mystery which is very fascinating, and the difficulty of finding any 

 theory which will satisfactorily account for such phenomena, endues 

 it with a singular interest. As Mr. Corbett says, the two principal 

 explanations so far advanced, are the retarded development, and the 

 migratory or blown-over, theories. As regards the first I think, with 

 Mr. Corbett, that it is untenable, if from no other reason than from 

 the probability that the only form, i.e., the pupa or ovum in which 

 such retardation could be possible, would, long before the expiration 

 of the decade which often elapses between two prolific seasons, have 



