The Chalcis Flies 



feeding externally upon the gall making larva, and there is a quite 

 large group known as the Elachistinae which are parasitic upon 

 caterpillars, the larvse of which feed also outside the skin of the 

 host insect. It is one of these which has been selected for our 

 typical life history. It was studied in the summer of 1878 in the 

 cotton fields of Alabama by Mr. E. A. Schwarz, but it has a north- 

 ward spread and is parasitic upon caterpillars of certain Geomet- 

 rid moths measuring worms. 



The adult parasite, which is a little black, shiny, four-winged 

 fly and which, when seen under a strong lens has a number of 



strong, dark colored bristles 

 upon its back, lays its eggs 

 to the number of from three 

 to fifteen in a group upon 

 the middle of the back of 

 the wriggling caterpillar. 

 Each egg is brown in color, 



Fig. 32. Larvae of Euplectrus comstockii, 



on cotton caterpillar. 

 (Fourth Report U. S. Ent. Com.) 



almost black before hatching, is elongate oval, strongly convex 

 above, and somewhat flattened beneath. The individual eggs 

 although laid in a group are sufficiently separated from each 

 other to allow for the development of the larvae. They hatch 

 about two days after being laid. The delicate egg shell splits 

 longitudinally in the middle of the back and discloses the white, 

 grub-like parasite larva, which gradually works the egg shell more 

 and more down the sides of its body where it remains visible as a 

 black line for some hours. As soon as the grub has freed its head 

 from the egg shell it pierces the skin of the victim, and thereafter 

 remains stationary with its head buried. As soon as it has 

 fairly begun to feed, the white color changes to a bright bluish 

 green, and the spiracles and the con- 

 strictions between the segments are 

 readily seen. The growth of this larva 

 is extremly rapid. In fact, we know 

 of no insect larva which has a more 

 rapid development. In midsummer it 

 reached full growth in three days from 

 the time of hatching. In September 

 this larval growth took four days. When full grown the para- 

 sitic grubs crowd each other, and if there are five or more of them 

 on a caterpillar they form a semi-globular lump of very striking 



58 



Fig. 33. Pupae of Euplectrns 



comstockii. (From frourth 



Report U. S. Ent. Com.) 



