Beetles 



257 



another near the base. When a live insect or other aquatic creature is caught 

 by the active larva its body is pierced by the mandibles and the blood sucked 

 through them into the mouth, the opening at the base just fitting, when the 

 mandibles are closed, into the corners of the small silt-like mouth. Both 

 larvae and adults are fierce and voracious, and the larger species attack and 

 kill small fish. In the middle states these beetles actually do much damage 

 in cjarp-ponds. The larva breathes through a pair of spiracles at the slender 

 tip of its body, which is thrust up to the air when it comes to the surface 

 of the water. When ready to pupate it leaves the water breathing now 

 also through six pairs of lateral spiracles and makes a rough cell in the 

 ground of the pond or stream bank. "The pupa state lasts about three 

 weeks in summer; but the larvae that transform in autumn remain in the 

 pupa state all winter." 



The larger of our common species belong to Cybister, Dyticus, and 

 allied genera. In Cybister the little cups on the under side of the tarsal 

 disks of the male are similar, and arranged in four rows. In Dyticus and 

 its allies the cups of the tarsal disks vary in 

 size. Fig. 349 represents a common species of 

 Dyticus. 



" The most common of the diving-beetles 

 that are of medium size belong to the genus 

 Acilius. In this genus the elytra are densely 

 punctured with very fine punctures, and the 

 females usually have four furrows in each wing- 

 cover." 



An interesting account of the habits 

 and special structures of the common large 

 European diving-beetle, Dyticus marginalis, 

 is given in Miall's Natural History of Aquatic 

 Insects, pp. 39-61. 



Smaller than the predaceous diving-beetles, 

 and readily recognized by their curious spin- 

 ning or circling, in companies, on the surface 

 of ponds or still pools in streams, are the 

 whirligig-beetles (Gyrinidae), common all over 

 the country. About forty species of these 

 ,beetles, varying in size from one-sixth to three-fourths inch in length, have 

 been found in North America, three-fourths of them belonging to the genus 

 Gyrinus. They are all of similar shape and steely blue-black in color, 

 and have the compound eye, on each side, wholly divided into an upper 

 and a lower part by the sharp lateral margin of the head. Like the 

 Dyticids, the whirligig-beetles breathe at the surface and carry air down with 



Fie. 350. 



FIG. 351. 



FIG. 350. Whirligig - beetle, 

 Dineutesemarginata. (Twice 

 natural size.) 



FIG. 351. Larva of whirligig- 

 beetle, Gyrinus marinus. 

 (After Schiodte; enlarged.) 



