NATURAL ENEMIES. 31 
These facts have particular interest as showing the effect of the 
parasite on the tissues of the host, and the most remarkable point 
perhaps is the formation of the giant cells with Trichasis, indicating a 
process of gall formation in these tissues analogous to the gall forma- 
tions produced in plant tissues by the presence of various insect- oi 
insect Larvae. 
Merisus destructor Say. (Fig. 2. 
This species, first characterized by Say as Ceraphron destructor, 
is doubtless the most universally important of the Hessian fly parasites, 
as it occurs not only throughout the American territory affected by the 
Hessian fly. but is known also in England and continental Europe. It 
has received notice from nearly every writer on the Hessian fly since 
its first description, and Dr. Riley has given a full statement of its 
synonymy and a discussion of its relation to subapterus, but no author 
has given us the knowledge of its early stages or the relation it bears 
in development to that of its host that is desirable in its practical treat- 
ment with reference to securing 
most advantage from its work. 
Riley says: 
The eggs of this parasite are with- 
out much doubt deposited in the half- 
grown larva- of the Hessian fly early 
in the spring, and in the more southern 
portions of the wheat belt there are in 
all probability two generations, the 
first issuing from the puparium in 
April and May and the second issuing 
all through the summer and fall. 
Many, judging from my experience in- Fig. -2.--M- istu desi uetor [from Riley). 
doors, hibernate in the pupal Btage ' 
within the Cecidomyid puparium and cut their way out the following spring. In 
the Xorth, however, there seems to be but one annual generation. 
In Iowa, as we have noted, there is an emergence of adults in early 
autumn, and either these must be able to deposit eg^ or else live over 
winter as adults, otherwise they must perish without providing for 
another generation. 
Marchal refers to this species certain larva- encountered in the larva- 
of Cecidoinyia, and characterizes them as glabrous and full, the larva- 
being very different in appearance from the normal occupants of the 
puparium. 
The following detailed description is by Dr. Riley, in Proceedings «>t 
the U. S. National Museum, volume 8, page 415: 
Male. — Length (average) 1.98 mm. Qreatesl width of fore wing, 0.62 mm. Antenna- 
long filiform, strongly pilose : fnniole joints subeqnal in width, decreasing Blightly in 
length from 1 to ti; joint 1 a little more than twice as long as broad; the olnb is 
nearly as long as the two preceding joints of the t'nnirle together, ovate, flattened 
on the sides, and acuminate at tij». The ocelli are lar^e ami prominent. Head and 
notum densely and rather tinelv punotate, the punctures <'ii the mesoscntellnm and 
