POLVMORPHA CLAVICORNS, SERRICORNS, ETC. 2 I 3 



best in the cooler waters of the earth. Lapland is one of the 

 parts of Europe richest in Dytiscidae, and the profusion of species 

 in the tropics compared with those of Europe is not nearly so 

 great as it is in the case of most of the other families of 

 Coleoptera. About 1800 species are at present known, and we 

 have rather more than 100 species in Britain.^ 



Series III. Polymorpha. 



Antennae frequently either thicker at the tip (clavicorn) or serrate 

 along their inner edge (serricorn) ; hut these characters, as 

 ivell as the number of joints in the feet and other 2}oints, are 

 very variaUe. 



Upwards of fifty families are placed in this series ; many of 

 these families are of very small extent, consisting of only a few 

 species ; other families of the series are much larger, so 

 that altogether about 40,000 species — speaking broadly, about 

 one-fourth of the Coleoptera — are included in the series. We 

 have already (p. 189) alluded to the fact that it is formed by 

 certain conventional series, Clavicornia, Serricornia, etc. united, 

 because it has hitherto proved impossible to define them. 



Fam. 10. Paussidae. — Antennae of extraordinary form, usually 

 two-jointed, sometimes six- or ten-jointed. Elytra elongate, hut trun- 

 cate hehind, leaving the pygidium exposed. Tarsi five-jointed. The 

 Paussidae have always been recognised as amongst the most 

 remarkable of beetles, although they are of small size, the largest 

 attaining scarcely half an inch in length. They are found only 

 in two ways ; either in ants' nests, or on the wing at night. 

 They apparently live exclusively in ants' nests, but migrate 

 much. Paussidae usually live in the nests of terrestrial ants, but 

 they have been found in nests of Cremastogaster in the spines of 

 Acacia fistulosa. They have the power of discharging, in an 

 explosive manner, a volatile caustic fluid from the anus, which is 

 said by Loman to contain free iodine. Their relations to the 

 ants are at present unexplained, though much attention has been 

 given to the subject. When observed in the nests they frequently 

 appear as if asleep, and the ants do not take much notice of 

 them. On other occasions the ants endeavour to drag them into 

 the interior of the nest, as if desirous of retaining their company : 



^ For classification and structure see Sharp, "On Dytiscidae," &•»'. Trans. E. 

 Dublin Soc. (2) ii. 1882. 



