H YMEN OP TER A. 
623 
the species are mostly of considerable size, and here belong the 
larger of the parasitic Hy menoptera. I n this fain ily the wings 
are furnished with several closed cells ; the fore wings have 
a stigma; and cells V, and 1st V 2 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 egg is forced 
when it is laid. 
TJialessa 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 antennas 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 
reached she deposits an 
Fig, 749.— Thalessa lunator. 
egg in it. The larva that hatches 
