Packard.] INSECTS OF THE POND AND STREAM. 143 
tens , from Nicaragua, they are a quarter of an inch long and 
two tracheal branches can be distinctly seen in each of them. 
The other abdominal spiracles are wanting in these three 
forms, though three pairs, according to Schiodte, are present 
on the thorax. 
There are a number of aquatic larvae which breathe by 
similar respiratory tubes. Such is the young of Dytiscus 
Fig. 105. 
Fig. loo. 
Eristalis larva. 
Fig. 107. 
Dytiscus larva. 
Merodon and larva. 
(Fig. 105). It has to rise to the surface for air, which is 
inspired through the bristle-like tubes at the end of the 
body. So with the larval Eristalis (Fig. 106), and its ally 
Merodon bardus (Fig. 107; a, larva) and Helophilus (Fig. 
108). These maggots frequent the most loathsome ditches, 
thick with mud and putrefying matter. Here they lie acting 
as scavengers and doing all they can to aid the State Board 
of Health. They wave their long flexible respiratory tubes 
aloft and drink in the pure air of heaven, not unmixed, per¬ 
haps, with the less than spicy odors emanating from their 
native puddle. Some of the Helophili frequent strongly 
brackish water, in fact almost purely salt water, while 
15 
