90 INSECTS. 



afford an opportunity for seeing something of the effects 

 of certain parasites of the cabbage butterfly, chief among 

 which is a little black ichneumon fly. This fly lays its eggs 

 in punctures in the skin of the larva. The eggs hatch out 

 little fly larvae, which feed upon the vitals of the butterfly 

 larva. The latter, if not killed by these parasites before 

 attaining its growth, becomes a chrysalis, inside which the 

 fly larvae continue their growth and transformations. 

 Finally there issues from the chrysalis, not a butterfly, 

 but a swarm of little black ichneumon flies. If half a 

 dozen chrysalides be kept, an infested one, or several, will 

 probably be found. If a chrysalis looks much blacker 

 than the others, it is surely infested; and, if placed in a 

 small vial, the ichneumon flies may be caught when they 

 issue forth. 



Immature Stages. Study the three preparatory stages. 

 I. The Egg. Study the egg to make out : 



1. Its attachment to the leaf. 



2. Its shape. 



3. Its surface markings. 



II. The Larva. Study the larva. Observe : 



1. The plainly distinguishable head. 



2. The cylindrical body, without distinguishable thorax 

 and abdomen. Count the segments. 



3. Three pairs of true legs on the first three body seg- 

 ments. These mark the portion of the body which is 

 later to become the thorax. 



4. Five pairs of false legs, called prolegs (abbreviated 

 from " prop legs "), borne on the sixth, seventh, eighth, 

 ninth, and last body segments. - 



5. Spiracles, a pair on each of the body segments ex- 

 cept the second, third, and the last two, opening at the 

 sides. The absence of spiracles on the second and third 

 segments marks these as the ones which are to bear the 



