AQUATIC INSECTS, 
J31 
AQUATIC INSECTS 
A considerable portion of the insect world live in or on the surface 
of still or running water, and are more or less specially adapted to the pe¬ 
culiarities of this mode of life. These insects are derived from terrestrial 
insects and there is no hard and sharp line between terrestrial and 
aquatic insects. We have, for instance, the predaceous Reduviid bugs on 
the earth which live also on mud and in the neighbourhood of water. 
It is but a small transition to the Hydrometridce , bugs which run on the 
surface of the water and which require very little modification, chiefly 
in the structure and motions of the legs. The aquatic carnivorous beetles 
are very closely allied to the land carnivorous beetles, the modifications 
mainly consisting of those necessary to enable the beetle to swim, to obtain 
air below water and to catch different prey. The Hydrophilidce include 
both land and aquatic forms in one family, and were our knowledge of 
past and present day insects greater, we might be able to trace the steps 
by which a land insect gave rise to aquatic forms. The aquatic Diptera 
are excellent examples, some living in mud, some in shallow water, some 
in deep water. We may suppose these to have more recently acquired 
the aquatic habit than such a homogeneous group as Odonata or Ephe- 
mendw which are now wholly aquatic and were probably derived from 
primitive land ancestors. 
Among aquatic insects, one of the most interesting features is the 
manner in which the air supply is obtained. Assuming that all aquatic 
insects are descended from terrestrial ancestors, and not from a single 
form which became aquatic, we would expect different groups to solve 
this problem in different ways and to find a great variety of devices to 
secure an air supply. In general mature insects obtain their air direct 
from the atmosphere, rising to the surface to do so, and there are among 
them fewer modifications in the respiratory system, possibly owing to 
the greater rigidity of the outer skeleton and the far smaller degree of 
plasticity of the adult constitution as compared with the larval. We 
may, therefore, consider the larval and pupal forms of aquatic insects, 
extremely briefly, solely from this point. Insects are commonly pro¬ 
vided with one or two thoracic spiracles, and a series of five to eight on 
the abdominal segments. This, the so-called holopneustic (2 thoracic) 
or perineustic (1 thoracic) system obtains in adults but not, so far as we 
are aware, in larvae. The first modification we find is the closure of all 
spiracles but the two terminal pairs, one near the head, one nearest to 
the tail (Amphipneustic). The closure of these spiracles is actual, but 
the spiracle remains, a tracheal vessel runs to it which contains no air 
as a rule. The larva of Pericoma (Psyehodidce ) and allied larvae are 
examples, and air is obtained by bringing either of the pairs of spiracles 
to the surface. A far commoner modification is the metapneustic one, 
where only the terminal abdominal spiracle persists in a functional state, 
being usually very large. A large number of insects exhibit this charac¬ 
ter in the larval state including Amphizoa , Dysticidce, most Hydrophili- 
