206 
HE1IIPTERA. 
To increase and multiply is their principal business in this 
period of their existence, if not the only one, and the natural 
term of their life ends when this is accomplished. In their 
previous states, however, they often pass a much longer time, 
the length of which depends, in great measure, upon the 
nature and abundance of their food. Thus maggots, which 
subsist upon decaying animal or vegetable matter, come more 
quickly to their growth than caterpillars and other insects 
which devour living plants ; the former are appointed to 
remove an offensive nuisance, and do their work quickly; 
the latter have a longer time assigned to them, corresponding 
in some degree to the progress or continuance of vegetation. 
The facilities afforded for obtaining food influence the dura- 
tion of life ; hence those grubs that live in the solid trunks 
of perennial trees, which they are obliged to perforate in 
order to obtain nourishment, are longer lived than those that 
devour the tender parts of leaves and fruits, which last only 
for a season, and require no laborious efforts to be prepared 
for food. The harvest-flies continue only a few weeks after 
their final transformation, and their only nourishment consists 
of vegetable juices, which they obtain by piercing the bark 
and leaves of plants with their beaks ; and during this period 
they lay their eggs, and then perish. They are, however, 
amply compensated for the shortness of their life in the 
winged state by the length of their previous existence, during 
which they are wingless and grub-like in form, and live 
under ground, where they obtain their food only by much 
labor in perforating the soil among the roots of plants, the 
juices of which they imbibe by suction. To meet the diffi- 
culties of their situation and the precarious supply of their 
food, for which they have to grope in the dark in their 
subterranean retreats, a remarkable longevity is assigned 
to them ; and one species has obtained the name of Cicada 
septendecivi , on account of its life being protracted to the 
period of seventeen years. 
This insect has been observed in the southeastern parts of 
