H YMENOP TERA. 



623 



the species are mostly of considerable size, and here belong the 

 larger of the parasitic Hymenoptera. In this family the wings 

 are furnished with several closed cells ; the fore wings have 

 a stigma ; and cells V^ and ist V^ are separate (Fig. 748). 



The largest members of the family belong to the genus 

 Thalessa. These are remarkable-looking insects, with long, 

 slender bodies and three long hairs at the end of the body. 

 Two of these hairs form a sheath for the third, which is the 

 ovipositor. This ovipositor, although apparently merely a 

 thread, is really composed of three pieces placed parallel, 

 one above and two below, and securely locked together. 

 Near the end of them are ridges like those on a file, and 

 between them is a passage through which the ^gg is forced 

 when it is laid. 



Thalessa lunator (Tha-les'sa lu-na^tor) is one of the 

 larger of our Ichneumon-flies. Its body is two and one 

 half inches long, and it measures nearly ten inches from 

 the tip of the antennae to the tip of the ovipositor. It 

 is a parasite of the wood-boring larva of the Pigeon Horn- 

 tail. When a female 

 finds a tree infested by 

 this insect she selects a 

 place which she judges is 

 opposite a Tremex-bur- 

 row, and, elevating her 

 long ovipositor in a loop 

 over her back, with its 

 tip on the bark of the 

 tree (Fig. 749), she 

 makes a derrick out of 

 her body, and proceeds 

 with great skill and pre- 

 cision to drill a hole into 

 the tree. When the 



TremeX-burrOW is Fig. t^^,— Thalessa lunator. 



reached she deposits an egg in it. The larva that hatches 



