346 APPLIED ENTOMOLOGY 



in the wood either on its way in, or during the withdrawal, that it cannot 

 be removed and the Thalessa dies. 



The egg left in the tunnel of the Horn-tail soon hatches and the larva 

 travels along the burrow until it finds the borer, to which it attaches 

 itself, and feeds upon its juices. It is probable that death of the Horn- 

 tail is not permitted as a result of this feeding, before the host has pre- 

 pared an outlet to the surface of the trunk for its escape, as it does not 

 seem likely that the adult Thalessa could tunnel its way out successfully. 

 Superfamily Cynipoidea (The Gall Insects). — This group of small 

 insects (Fig. 362) includes species having very diversified habits, but the 

 majority of them pass their early stages within abnormal growths on 



plants, called galls, which develop in connection 

 with their presence. Some insects other than 

 those of this group also produce galls, particu- 

 larly many of the dipterous family Itonididse, 

 but the greater number of the more noticeable 

 galls are produced by Cynipids. 



The cause of the production of the gall has 

 ^ , , been much discussed, some investigators claiming 



Fig. 362. — Example of a . , <• i • , < i_ 



Gall insect {Trigonaspis that at the time the female msect punctures the 

 megaptera Panz.), about plant for egg-laying she also injects a little 



twice natural size. {Reduced . . i i • i ,• i . ;i 



from Henneguy.) poison mto the wound which stimulates the 



plant cells of that region to grow in an abnormal 

 manner. But the general belief now seems to be that the larva when 

 it hatches gives the stimulus for this abnormal growth, either by its 

 presence as a moving body, by its gnawing, or by its pouring out of 

 irritating fluids. 



The gall includes either one larva or many, according to the species 

 concerned. It stops its growth about the time the larva finishes feeding, 

 and dries, forming a protective covering within which the insect pupates 

 and escapes subsequently by gnawing its way out. 



In some species such an adult will attack a totally different kind of 

 plant from the one it itself fed upon and the gall produced will bo entirely 

 different from the other. Adults from such galls will deposit their eggs 

 in plants of the first kind, however, giving us a series of generations in 

 which two different kinds of plants alternate in supplying food. This 

 may be complicated by one generation consisting only of females, the 

 other evidently being derived by agamic reproduction (parthenogenesis). 



Galls may occur on roots, stems, twigs or leaves, and the type of gall 

 produced is always the same on any one kind of plant, for the same spe- 

 cies (Fig. 363), so that a student of the subject can tell from the gall 

 alone, the species which produced it, in nearly every case: one found on 

 oak leaves is nearly an inch in diameter, globular, with a parchment-like 

 covering, and is often called an "oak apple." Within the outer covering 



