918 AN INTRODUCTION TO ENTOMOLOGY 



Megarhyssa lundtor, which was formerly known as Thalessa lund- 

 ior, is one of the larger of ichneumon-flies (Fig. 1 153). It is a parasite 

 of the wood-boring larva of the pigeon horn-tail Tremex columba. 

 When a female finds a tree infested by this borer she selects a place 

 which she judges is opposite a Tremex-burrow, and elevating her 

 long ovipositor in a loop over her back, with its tip on the bark of the 

 tree (Fig. 11 53), she makes a derrick out of her body and proceeds 

 with 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 Megarhyssa 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 a hole already made 

 by the Tremex. Sometimes the adult Megar- 

 hyssa, like the adult Tremex, gets her ovi- 

 positor wedged in the wood so tightly that 

 it holds her a prisoner until she dies. 



Among the more common of our larger 

 Ichneumon-flies are those of the genus 

 Ophion (Fig. 1 1 54) ; these have yellow bodies. 

 One species infests the caterpillars of the 

 Fie ii^±— Ophion Polyphemus-moth and only a single egg is- 

 laid within each victim. The caterpillar lives 

 until it spins a cocoon, 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, Agrothereutes extremdtis, 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 TRIGONALID^ 

 The Trigonalids 

 In this family and in the three following families there is a distinct 

 cell between the costa and the radius of the fore wings; these insects 

 differing markedly in this respect from the braconids and the ichneu- 

 mon-flies. In the trigonalids and stephanids the insertion of the ab- 

 domen is normal, differing in this respect from the Aulacidje and the 

 Gasteruptionidas. In the trigonalids there are two or three closed sub- 

 marginal cells in the fore wings, which distinguishes them from the 

 Stephanidae. 



This family is represented in our fauna by only a small number of 

 species. Some species infest nests of social wasps, some have been 

 bred from pupas of Lepidoptera, and one was bred from the cocoon of 

 an Ophion in a cocoon of Telea polyphemus. 



Family AULACID^ 

 The Aulacids 

 In this family the abdomen is borne on the dorsal surface of the 

 propodeum far above the middle coxas; in this respect these insects 



