PACKAun] INSECTS OF THE POXD AND STREAM. 143 



tens, from Nicaragua, the^' are a quarter of fin 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 thi'ee pairs, according to Schiodte, are present 

 on the thorax. 



There are a number of aquatic larvse which breathe by 

 similar respiratory tubes. Such is the young of Dytiscus 



Fig. 105. Fig. lOG. 



Eristalis larva. 

 Fig. 107. 



Dytisous larva. 



Mcrodon 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 Ihe 

 body. So with the larval Eristalis (Fig. 106), and its ally 

 Merodon hardus {¥\g. \Q1 ; a, larva) and Ilelophilus (Fig. 

 lOS). These maggots frequent the most loathsome ditches, 

 thick with mud and putref3'ing 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 Ilelophili frequent strongly 

 brackish water, in fact almost purely salt water, while 



15 



