366 



ZOOLOGY. 



metamorphosis is either complete or incomplete. Many of 

 the species are aquatic. There are ten existing families, 

 and certain species of most of these families resemble or 

 anticipate the higher orders that succeeded them in time. 

 Thus the caddis-fiies would be easily mistaken for small 

 moths ; the Mantis-pa mimics Mantis, the Psocus mimics 

 the plant-lice, and the white ants which appeared in the 

 Coal Period foreshadow the genuine ants, which first ap- 

 peared in the Tertiary Period, wonderfully in form as well 

 as in the extreme difEerentiation of the individual, involv- 

 ing a complicated colony with numei'ous members of the 

 entomological body politic. 



Fig. 326.— Pupa of a Drag- 

 on-fly {EschiM) 



Fig. ZIQ .—Agrion, natural size, and a, its 

 larval gill, much enlarged. 



As many of the larval Neuroptera live in the water, the 

 various forms of external gills are of high interest. In the 

 case-worms, which construct tubes of sand, bits of leaves, 

 etc., the gills are slender filaments permeated by air-tubes 

 and situated along the side of the body. In the larval and 

 pupal dragon-fly (Fig. 325) the water passes into the intes- 

 tine, where there are folds rich in minute tracheal branches. 

 In the smaller dragon-flies, such as Agrion (Pig. 336), there 

 are three long, leaf-like gills, interpenetrated by air-tubes. 



