18 THE HESSIAN FLY IN THE UNITED STATES. 
antennae, eyes, legs, wings, and other organs which are not represented 
in the larva, but which appear in an advanced form when the change 
to pupa takes place. For these changes Marchal states his determina- 
tion that moisture is absolutely necessary. It is during the early part 
of this stage that the reversal of position takes place that has been 
mentioned in connection with the description of the sternal spatule. 
After the change to the pupa stage has occurred there is a variable 
period during which the insect remains within the pupariura, and its 
issue doubtless depends on favorable temperature. The end of the 
puparium is forced of, and the pupa, by means of its pointed rostrum 
and movements of the body, extricates itself from the case and pushes 
its way out of the leaf sheath to freedom. Sometimes, according to 
Enock, it may cut its way directly through the leaf to the outside. 
The pupal sheath then quickly splits and the imago issues, leaving in 
the pupal moult a perfect but extremely delicate cast of the pupal 
form. 
The lifetime of the adult is short, lasting only the few days necessary 
to the processes of mating and deposition of eggs. If delayed in 
these, it may probably survive several days; if they are accomplished 
promptly, its life may be measured by hours rather than days. Copu- 
lation is said to take place usually very shortly after emergence, and 
generally toward the middle of the day, lasting but a few seconds, 
and one male is capable of fertilizing several females. Fertilization 
appears to be necessary to the development of the eggs, as records 
show that unfertilized eggs fail to develop. It may be assumed, there- 
fore, that parthenogenesis is impossible as a normal feature, and that 
in case isolated females issue in new territory there would be scarcely 
a possibility of their establishing a colony. 
' We come now to the consideration of the number of annual genera- 
tions and the questions of acceleration and retardation, which seem to 
me so closely related as to be best discussed in one connection. This 
subject presents especial importance from the practical side, as the 
most important means of control available to the cultivator are depend- 
ent upon it. 
ANNUAL GENERATIONS. 
An insect that can adapt itself to all the climatic conditions and crop 
variations between North Dakota and Texas must necessarily be able 
to vary its life history in no small degree. The Hessian fly presents 
variations not only in the number of broods, from one to possibly five 
or six, depending upon latitude, but, by acceleration or retardation, 
under conditions peculiar to each year, it may appear earlier or later 
and in a greater or less number of broods in the same locality. This 
variation naturally enhances the difficulty of stating life history details 
with precision and making recommendations as to particular dates on 
which to adopt measures of control. 
The earlier writers detailed two broods for each year, the adults 
