DYTISCUS MARGINALIS. THE GREAT WATER BEETLE. 153 
the thorax being known by the name of coxa. The plate joining 
the coxa to the femur, or more correctly the plate covering the 
junction of the two, is known as the ¢rochanter. The anterior pair 
of legs is fitted with an apparatus of a very wonderful kind in the 
Dytiscus marginalts, for not only do the ¢asrv# end in a pair of 
formidable claws, but a circular enlargement is found fitted with 
two large suckers, these being supplemented by a large number of 
smaller ones having trumpet-shaped mouths. The effect of these 
is easily seen. When once the beetle seizes a fish or other object 
which he wishes to kill, the suckers are at once applied and he is 
as it were glued to his prey, and no amount of wriggling will shake 
him off. This portion of the insect is shown in fig. 26, and its 
structure will well-repay attention. At the upper surface of the 
head, the protecting portion in front is known by the term cypeus, 
or shield, the upper central portion is the vertex, or crown, the 
circular parts represent the eyes, while the back part is termed the 
occiput. On the under side of the head the eyes are represented 
by circles, while the depressions in front show the insertion of the 
antenne. ‘The front portion of the upper side of the thorax is 
termed the azterior margin, the angular points in front being 
known as the anterior angles ; the posterior margins and posterior 
angles being nearer the abdomen. ‘The main plate of the proster- 
mum or under surface of the thorax is known as the sternum, while 
