THE TURNIP-FLY. 



495 



The true Saw-flies are known by the curious piece of animal mechanism from which 

 they derive their name. The females of this family are supplied with a pair of horny 

 saws, placed side by side on the lower extremity of the abdomen. 



These saws are of various forms, according to the particular species to which they 

 belong, and may be seen even in the dried specimens, the top of their sheath slightly 

 projecting, and their shapes plainly visible after the removal of a portion of the 

 abdomen. When taken from the insect and placed under the microscope, they pre- 

 sent a very pretty appearance, owing to the gently-curved ribs with which their sides 

 are strengthened and decorated. The saws act alternately, one being pushed forward 

 as the other is being retracted. Their object is to form a groove in some plant, in 

 which the eggs of the mother insect can be deposited, and wherein they shall find a 

 supply of nourishment in order to enable them to complete their development ; for it 

 is a most remarkable fact that, after the egg is deposited in the groove, it rapidly 

 increases in size, obtaining twice its former dimensions. 



In the genus Cimbex, of which an example is given in the illustration, the larvae 

 possess twenty-two feet, and have the power of discharging a translucent greenish 



Cimbex temorato. 

 Rbyssa persuasorla. 



Urocerus gigas. 

 Ichneumon grossarlus. 



fluid from certain pores placed on the sides of the body just above the spiracles. 

 This feat they can repeat six or seven times in succession. When they have eaten 

 their way to the next stage of existence, they spin a cocoon of a brownish color and of 

 a stringy, tough consistency, and either suspend it to the branches of the tree on 

 which they have been feeding, or hide it under fallen leaves. In this cocoon they 

 remain for a comparatively short time, and then emerge as perfect insects. 



The terrible TURNIP-FLY (Athalia centijolia) belongs to this family. The larva of 

 this species is popularly called the Nigger, on account of its black color. A species of 

 Athalia may be seen in the engraving on page 463, and is the smallest of the three flying 

 insects at the upper part of the illustration. The larva of this species feeds upon the 

 various cabbages, eating away the whole of the soft green parts of the leaves, and only 



