386 INSECTS ABROAD. 



At the eud of the abdomen, and on the under side, is a pair 

 of horny plates, each edged with teeth like those of a saw. The 

 plates curve slightly upwards, and each slides backwards and 

 forwards in a groove sunk in another horny plate, which thus 

 serves both as a support and a guide. As the saw has to be 

 employed in cutting green wood, it cannot be made simply by a 

 Hat plate edged with teeth, like the saws with which dry timber 

 is cut. Each saw is rather thick, and the teeth are arranged 

 very much like those on the back of a sapper's sword. On a 

 closer examination with the microscope the teeth are seen to be 

 further elaborated, each separate tooth being a conical cutting 

 instrument with some eight or ten sharp edges. And in order to 

 secure a wide groove, or " kerf " as carpenters call it, the edges 

 of the saws are furnished with a sharp zigzag projection, some- 

 thing resembling a knife-blade that has been folded in this form , 

 /\/\/\/\/\/, and fixed with its back against the side of 

 the saw. 



When used the saws are worked alternately, so that a wide 

 groove is cut in a very short time. Having thus performed the 

 office of the saw, they then assume that of the ovipositor, the 

 blades diverging so as to permit the passage of an egg, which is 

 then placed in the groove. A small drop of some irritant fluid 

 follows the egg, and causes the edges of the groove to swell, and 

 so enclose the egg firmly between them. Here it is kept until 

 the time for hatching, when the young grub crawls out, and 

 betakes itself to the leaves for its subsistence. In our own 

 country these insects are often very injurious, the worst of them 

 all being the dreaded Turnip Fly (Athalia centifolice), whose 

 grey-black grubs are so well known under the name of Niggers. 



We will take a few specimens of these curious insects, and 

 mention a few of their peculiarities as we proceed. The species 

 which is shown on the next page is found in the neighbourhood 

 of Hudson's Bay, and derives its specific name from the shape 

 of the clypeus, which is triangular in the male sex. The head 

 is black and shining, and the thorax and base of the abdomen 

 are also black, but covered with short grey hairs. The rest of 

 the abdomen is dark chestnut with a yellowish tinge. The 

 thighs are black, and the rest of the legs yellow, and the wings 

 are all yellow and glossy. 



This is a very large genus, and in the British Museum are a 



