i68 



INSECTS. 



when it becomes vitiated can be squirted out again either gently or with con- 

 siderable force. When it is suddenly and violently expelled, it serves to propel 

 the insect forwards at a rapid rate. The larvae live about ten or twelve 

 months, during which time they undergo several moults; rudiments of wings 

 appearing some time before the final transformation. When this is about to 

 take place, the larva leaves the water by climbing the stem of a plant, or to 

 some other dry spot. As the time approaches, its eyes, which were before dull 

 and opaque, become bright and transparent. Its skin dries up, and soon begins 

 to crack along the middle of the thorax: the thorax appears through the cleft, 

 and swelling up causes it to extend ; the head is next disengaged, and the legs 

 are then drawn out of their sheath. The insect now throws its head farther 

 and farther back, and by this means gradually frees the hinder part of its 

 body, with the exception of the last few segments which still remain enclosed in 

 the larval skin. After a while, it suddenly bends its body forwards, grasps the 



ft n 





12 3 4 5 



LIFE-HISTORY OF DRAGON-FLIES. 



1, Larval skin of a dragon-fly ; 2, Larva with its mask exserted ; 3, Lihelhda depressa ; 4, Advanced larva 

 of a libellula ; 5, The same about to undergo its final transformation. 



sides of the sheath with its legs, and, doubling up its abdomen, finally extricates the 

 rest of its body. 



Draeon-nies are divided into three families, of which the first two have more 

 in common with one another than with the third. The Libellulidce are dis- 

 tinguished by their comparatively stout bodies ; by the size of their eyes, which 

 cover almost all the sides of the head, and very nearly meet on its crown ; and 

 by the structure of their lower lip. in which the median terminal piece is short 

 and slightly divided at the end, while the very broad palps spread out and overlap 

 it in front. The last character is useful in distinguishing the Libellulidce from the 

 next family, which in many respects they resemble. Their larvae breathe by 

 means of internal gills, and have a mask which is hollowed out on the inner side, 

 and somewhat resembles a helmet. Members of this family are found in most 

 parts of the world, and about twenty species occur in Europe. The jEschru dee 

 have eyes even larger than those of the Libellulidce. The end piece (ligula) 



