508 NATURAL HISTORY OF ARTHROPODS. 



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 The family Ctstipid^ comprises forms known as gall-flies. In these the abdomen 

 is usually furnished with a short peduncle and is much compressed. The second or 

 the second and third segments (the first forms the peduncle) are greatly developed. 

 The remaining segments (usually five in number) are short, and each is more or less 

 covered by the preceding segment. Concealed within these segments is the long, 

 partially coiled, very slender ovipositor, which arises near the base of the abdomen. 

 The wings of the gall-flies have comparatively few veins, and the fore wing lacks the 

 stigma; some foi-ms are wingless. The antennae are not elbowed, and consist of from 

 thirteen to sixteen joints. The larvffl are maggot-like, and without caudal opening to 

 alimentary canal. 



These insects are termed gall-flies because the majority of the species live within 

 galls. But the student must bear in mind that not all the species of this family are 

 developed in galls, and that not all galls are produced by members of this family. A 



gall is an abnormal vegetable growth caused by an 

 insect, and within which one or more insects live and 

 derive their nourishment. The common oak-apples 

 are familiar examples of galls. 



If one of these galls be cut open at the proper time, 

 there will be found at the centre a cell containing a 

 grub-like larva. This larva derives its nourishment 

 from the substance of the gall. Many galls are com- 

 pound, a large number of cells being united to form a 

 single gall. The mossy rose-gall, so common on the 

 wild rose, is an illustration of this. 



Many species of gall-flies undergo their transforma- 

 tions within the gall ; while in other species the full- 

 grown larva leaves the gall and enters the ground to 

 transform. Each species of gall-fly infests a special 

 Fig. 633. — Gall of Miwdites roscB on part of one Or more particular species of plants. And 



the gall produced by each species of insect is of definite 

 form., Thus^ when an entomologist who has studied these insects sees a familiar gall, 

 he knows at once what species of insect produced it. 



The manner in which galls are produced has been the subject of much speculation. 

 The female gall-fly, as already stated, is furnished with an ovipositor. By means of 

 this instrument she punctures the surface of that part. of the plant where the egg is to 

 be laid. Now it has been supposed that at the time the egg is laid there is propelled 

 into the wound of the plant a small quantity of an irritating fluid, the action of which 

 upon the plant in some way causes an abnormal production of cellular tissue. On this 

 hypothesis the differences between the galls of different insects was explained by sup- 

 posing that the fluid produced by each species of insect had peculiar properties. It 

 has been shown that in many cases this cannot be true. It is, however, still held for 

 certain insects. Thus, the wound made by a certain saw-fly (JVematus vallesnierii) 

 in the leaves of Salix amygdalina " causes an abundant foraiation of cells, and the 

 gall thus formed attains its full growth at the end of a few days, before the larva has 

 escaped from the egg." " In the Cecidomyidse, on the contrary, the manner in which 

 the eggs are laid show clearly that it is the larva which causes the formation of the 

 gall. The same is the case with the Cynipidse. No effect is produced until the larva 

 is hatched. Trigonaspis crustalis lays its eggs in May, and the larvae do not hatch 



