624 THE STUDY 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 OpJiion (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, 

 Fig. 75 o.-o/Aion. but 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 Stephanids {StepJi'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 1st V 2 of the fore wing, 

 but differ in having a cell between veins I and IIL 



