AQUATIC INSECTS. 
133 
6 4 Blood-gills ? ? are gills as opposed to tracheal gills* since only the 
body-fluid circulates in them and no tracheae enter them, or if they do,, 
do not contain or carry air ; the gills of Pelohius P Hydrocyphon and some 
Chironomus, the rectal pouches of Macronema . the gills of some young 
Phryganeids and Ephemerids are of this class, though in the last there 
is little real distinction from tracheal gills. 
There are a small number of insects in which air enters the body 
cavity and this is so extraordinary a phenomenon that though we know 
of it in only two insects, we mention it here. It deserves fuller investig¬ 
ation. Another peculiar method is found in larvse which take air into 
the alimentary canal, either swallowing it as does one aquatic larva, or 
as Odonata do, at the hind end ; this is often seen in the latter in captiv¬ 
ity, and is simply a modification of the rectal gill. 
Finally, there are the insects which contain a red pigment allied to 
or identical with Haemoglobin, the constituent of man’s blood that 
carries oxygen in weak combination from lung to tissue and Carbon 
Dioxide to the lung. Chironomus is the familiar example, found in 
every Indian tank, and we use this generic name in a very broad sense 
to include many forms allied to Chironomus but not identified. 
For the benefit of the student we attach the table of modifications 
mentioned above. 
I. Tracheal : 
1. Stigmata: 
a Holopneusiic, 
h Peripneustiai 
c Ampkipneustic. 
d Metapneustic. 
■ e Propneustic. 
2. No Stigmata : 
/ Tracheal gills, main. 
g Tracheal gills , accessary . 
h Rectal gills. 
II. Without tracheae : 
i Skin, wholly or in part. 
j Blood-gills. 
k Entrance of air to body. 
i Entrance of air to gut. 
- m Pigment. 
Summary of aquatic families .—The'following review does not pre¬ 
tend to mention every aquatic form or group, but contains the majority, 
and probably every important family. 
Aptera include aquatic forms living on the surface of water. Pod- 
mince are known to have this habit and, were we to include the marine 
forms, the well-known Anurida could be cited. Aquatic Orthoptera, 
