INTRODUCTION. XXV11 



sites, chiefly of the family Tachinidce, which are similar in 

 their habits to the Ichneumons ; and the maggots produce flies 

 like blue-bottles or house-flies, but with very bristly bodies. 

 These parasites consume the bodies of their victim, so that it 

 dies either in the larva state or as a pupa, the parasites either 

 undergoing their transformations within the empty skin, or 

 emerging from it, and forming their own cocoons round it. 

 Occasionally, if the larva has only been attacked by one or 

 two of the smaller parasites, it attains maturity, but is more or 

 less crippled. Generally speaking, these parasites are more 

 or less restricted to certain species of "hosts," as they are 

 technically called, and attack no others ; but many parasites 

 will attack a variety of different species almost indiscriminately. 

 On Plate II., fig. 8, we have figured one of the parasites 

 which attack the common Cabbage Butterfly ; it is Microgaster 

 glomeratus. Linn., greatly magnified. " The size is very 

 diminutive, the largest specimen seldom exceeding two lines 

 in length. The general colour of the body is deep black, and 

 the legs reddish-yellow. The wings are somewhat longer than 

 the body and pubescent, each of the upper pair having a tri- 

 angular black spot near the middle of the anterior margin (the 

 pterostigma), three discoidal cells, and a triangular areolet, 

 rather imperfectly formed. The abdomen is furnished with an 

 ovipositor, consisting of two flat valves, and a curved horny 

 sheath, terminating in a point. The use of this instrument 

 is to pierce the skin of the caterpillar, and to form a conduit 

 for conveying the eggs into the hole thus prepared for their 

 reception. When the fly has selected a caterpillar fitted for 

 her purpose, she alights upon its back, and plunges her weapon 

 into its body, chiefly at the incisions of the segments, deposit- 

 ing an egg at every insertion. This operation is repeated till 

 no fewer than thirty or forty eggs are sometimes laid in the 

 body of a single caterpillar. These are soon hatched in their 



