24 THE ABDOMEN. 



(elytra], and in most of those insects in which they 

 attain the hardest consistency, they meet in a straight 

 line down the back, and usually cover exactly all the 

 upper part of the abdomen, so that their owner walks 

 about in a complete suit of armour. At their base, 

 however, they usually leave a portion of the back of 

 the mesothorax uncovered, and this, which forms a 

 triangular horny plate, is called by entomologists the 

 scutellum. 



Although the majority of our favourites are so well 

 furnished with wings, there are a few insects in almost 

 all the larger groups which are entirely destitute of 

 these organs, whilst in others the hinder pair only is 

 deficient. The latter is the case also in one entire 

 order of Insects, the order to which our common 

 House-fly belongs, in which, as any one may convince 

 himself with very little trouble, there is but a single 

 pair of membranous wings. Behind the wings, in 

 these insects, we find a pair of slender knobbed fila- 

 ments, called the halteres or balancers, and these are 

 regarded by many entomologists as the vestiges of the 

 posterior pair of wings. 



The Abdomen. The consideration of the general 

 structure of the abdomen need not detain us long. 

 It consists normally, as already stated, of nine seg- 

 ments, but of these the whole are rarely visible, 

 some of them being generally reduced in size, modified 

 in form, and concealed within the others. In most 

 insects the union between the segments is effected by 

 a soft flexible skin, and for the protection of this, the 

 hinder margin of each segment overlaps the base of 

 the succeeding one, so that the soft fold of skin 

 enables the segments to slide in and out, something 

 like the joints of a telescope. This is the case in 



