624 
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 Ophion (O'phi-on) (Fig. 
AvKj 750) ; these have yellow bodies. They 
KfcKj infest the caterpillars of the Polyphe- 
mus-moth, and only a single egg is 
laid within each victim. The cater- 
* l * l J pillar lives until it spins its cocoon, 
Fig. 750 .-ophion. but <j oes no t 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 extrcmatis (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-flics 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 {Stcph f 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 a of the fore wing, 
but differ in having a cell between veins I and III. 
