218 NATURAL HISTORY OF BHOMPSTON POND. 



appearance ; the two external egg -sacs (k), each containing 

 some fifty or sixty eggs, can be distinguished even with the 

 naked eye. The males have no egg-sacs, but the antennules 

 are hinged, and act as prehensile organs. Several larval 

 stages are often taken together with the adult forms. The 

 nauplius (figs. 54, 55), larval (youngest) stage, possesses a 

 single eye (a) and three pairs of appendages ; the first pair (b) 

 develops into the antennules of the adult, the second pair (c) 

 is biramous and becomes the antennae, while the third pair 

 (d), also biramous, becomes the mandibles of the adult. The 

 relatively short body terminates in two setae. A fourth pair 

 of appendages soon appear, the future maxillae, and later on 

 three other pairs develop, the larva being known at this stage 

 as a metanauplius. As development proceeds the remaining 

 appendages appear, and the adult form is gradually acquired. 



Canthocamptus minutus (fig. 56), an elongated reddish 

 copepod, in which the cephalo thorax (a) gradually merges into 

 the abdomen (b), is closely related to Cyclops. It has a single 

 median eye, but has no heart. The female has a single ventral 

 egg-sac (c) and eight-jointed antennules, while the male has 

 no egg-sac, and antennules of only seven joints, which it uses 

 together with the antennae for holding the female. 



In insects the respiratory air is conveyed directly to the 

 tissues through invaginations of the chitinous covering of the 

 body in the form of finely branched tubes called tracheae, 

 which usually open to the exterior by stigmata (spiracles), 

 Plate 4, fig. 1, is a photomicrograph of a portion of a trachea 

 dissected from the abdomen of the larva of Dytiscus (fig. 64. c), 

 a carnivorous water-beetle. The photograph shows the some- 

 what spiral cuticular thickenings of the wall of tha trachea, 

 which prevent it from collapsing when slightly compressed. 



The larva of Cloeon (a May fly) (fig. 57), with long antennae, 

 which occurs in the mud at the bottom of the pond, affords 

 an interesting exception ; here the tracheae do not open to the 

 exterior, but they take up oxygen from the water through six 

 pairs of double tracheal gills (a). These lateral respiratory 

 processes of the anterior six abdominal segments are 



