THE STUD Y OF INSECTS. 



from this egg creeps along this burrow until it reaches 

 its victim, and then fastens itself to the horn-tail larva, 

 which it destroys by sucking its blood. The larva of 

 Thalessa when full grown changes to a pupa within the bur- 

 row of its host, and the adult gnaws a hole out through the 

 bark if it does not find a hole already made by the Tremex. 

 Sometimes the adult Thalessa, like the adult Tremex, gets 

 her ovipositor wedged in the wood so tightly that it holds 

 her a prisoner until she dies. 



The most common of our larger Ichneumon-flies belongs 



to the genus OpJdon (O'phi-on) (Fig. 

 750) ; these have yellow bodies. They 

 infest the caterpillars of the Polyphe- 

 mus-moth, and only a single egg is 

 laid within each victim. The cater- 

 pillar lives until it spins its cocoon, 

 b ut does not change to a pupa. The 

 Ichneumon larva when full grown spins a dense brownish 

 cocoon within the cocoon of the caterpillar. Another smaller 

 Ichneumon-fly, Cryptus extrematis (Cryp'tus ex-tre-ma'tis), 

 infests the same caterpillar, but more than one egg is laid in 

 a caterpillar by the female. We have bred thirty-five of 

 these Ichneumon-flies from one caterpillar. The larvae of 

 this species also spin their cocoons within the cocoon of their 

 host. 



Family STEPHANID.E (Ste-phan'i-dae), 

 The StepJuinids (Steph'a-nids). 



This family includes only four North American species, 

 and all of these are rare. They resemble the Braconids in 

 lacking the vein between cells V, and ist V, of the fore wing, 

 but differ in having a cell between veins I and III. 



