96 BRITISH INSECTS 



I 



families, which fall naturally into two groups. In the 

 first group 1 the antennae are slender and thread-like, 

 composed of at least seven joints, usually of many 

 more. This group comprises the midges, gnats, crane- 

 flies, and their allies. The most interesting families 

 are the following : 



The gall-midges 2 are tiny, gnat-like flies, whose 

 family name has reference to the fact that many of 

 the species, as larvae, affect the tissues of plants in 

 such a way that abnormal growths or galls are pro- 

 duced. For instance, the midge called Cecidomyia 

 saliciperda lays her eggs in rows on the bark of willow 

 stems, and the minute grubs, on hatching, burrow 

 inwards. They so irritate the cambium i.e., the 

 actively growing tissues of the stem that eventually 

 a spindle-shaped swelling is produced ; and in the end 

 the bark ruptures and hangs down in shreds. When 

 a stem several inches in circumference is attacked by 

 numerous midges, a very large gall-structure may 

 result, such as the specimen shown in the photograph 

 reproduced on Plate XL, which measures nearly a 

 foot in length. The tiny punctures scattered over the 

 surface bear testimony to the vast swarms of midges 

 that must have issued from this great gall. Many other 

 midge-caused galls, some of them curiously shaped 

 and prettily coloured, may be found on different kinds 

 of plants ; but membership of this family by no means 

 necessarily implies the gall-making habit. The larvae 

 of many Cecidomyids e.g., the notorious Hessian- 

 fly 3 do great harm to plants by secreting themselves 

 in the tissues and sucking the sap, the process being 

 unaccompanied by any abnormal growth on the part 

 of the vegetable cells. The larvae of other species 



1 Nematocera. a Cecidomyidce . 3 Cecidomyia destructor. 



