COLEOPTERA. PARNIDiE. 115 



ternal and internal anatomy of this genus. The family comprises the 

 single genus Heterocerus. 



The French entomologists regard the variations exhibited by the 

 European individuals of this genus as indicating only varieties of a 

 single species. 



Fabricius describes a species from Tranquebar, and Harris mentions 

 an American species ; Dejean unites the North American species, H. 

 pallidus Say, with the common European species. 



The family Parnid.^ MacLeay (or Macrodactyla Latr.'), like the 

 preceding, is composed of minute subaquatic insects {fig. 7. 7. Dry ops 

 Dumerilii), distinguished by their narrow and unarmed tibiae and 

 moderately long tarsi, which are formed for walking alone, the termi- 

 nal joint being very long, and armed with two strong claws at the 

 tip {fig- 7. 9. tibia and tarsus of Parnus). The body is oval or ob- 

 long, and more or less convex, with the posterior part of the thorax 

 as broad as the base of the elytra. The antennae are variable in their 

 form, but are always short ; the upper and lower lips are large, the latter 

 equalling the mentum in size; the palpi are very short, with the termi- 

 nal joint rather thicker and longer than the preceding ; and the man- 

 dibles are robust, notched at the tips, with the inner surface ciliated. 



The mode of respiration of these insects has been the subject of 

 controversy ; M. Audouin considering it to be effected as in the 

 Aepus fulvescens, whilst M. Wesmael, at the Academy of Sciences at 

 Brussels, has observed that the upper surface of the body of Elmis is 

 glabrous, and consequently unable to retain a coating of air ; the under 

 surface is, however, furnished at each side with a broad longitudinal 

 band, contiguous to the lower margin of the elytra, formed of a silky 

 coat ; the body of the Parnus is, on the contrary, entirely villose, so 

 that it is probable that the entire body is surrounded with a coat of 

 air when the insect is under water. (See A7in. Soc. Ent. Fr. 1835, 

 p. xl.) 



This family comprises two divisions, which have been formed by 

 Mr. Stephens into families ; but as they agree in many of their essen- 

 tial characters, I have followed Latreille and MacLeay in considering 

 them as constituting a single family, composed of two subfamilies, viz. 

 the Parnides and the Elmides. 



The distribution of the groups composing this and the preceding 



I 2 



