COMMENSALISM AND PARASITISM .343 



a Tremex burrow, and, elevating her long ovipositor in a 

 loop over her back, with its tip on the bark of the tree (Fig. 

 208), she makes a derrick out of her body and proceeds with 



FIG. 206. The adult ichneumon fly. The lines indicate natural dimensions. 



great skill and precision to drill a hole into the tree. When 

 the Tremex burrow is reached she deposits an egg in it. 

 The larva that hatches 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 burrow of its host, and the adult gnaws a hole 

 out through the bark if it does not find the hole already 

 made by the Tremex. 



277. Degeneration through quiescence. If for any other 

 reason animals should become fixed, and live inactive or 

 sedentary lives, they would degenerate. And there are not 

 a few instances of degeneration due simply to a quiescent 

 life, unaccompanied by parasitism. The Tunicata, or sea- 

 squirts (Fig. 210), are animals which have become simple 

 through degeneration, due to the adoption of a sedentary 



