THE STUDY OF INSECT LIFE 13 



circulation and the number of pulsations of the 

 dorsal vessel varies with the atmospheric tem- 

 perature, nearly ceasing at freezing-point. 



In connection with the classification of insects, 

 there are still many knotty points which can 

 only be conclusively settled by results to be 

 obtained by future embryological research, con- 

 sequently considerable difference of opinion and 

 want of unanimity on the subject exists. How- 

 ever, much has been accomplished, and the once 

 long and arbitrary series of Orders into which 

 the insects were divided have been considerably 

 modified. The insects are now generally divided 

 into the following seven Orders : 



1. Order Coleoptera, comprising the Beetles, 

 a very numerous group of insects, possessing 

 two pairs of wings, the front pair (elytra) being 

 horny and dense, meeting in a straight line down 

 the back when closed, and forming a protective 

 case to the membranous posterior pair. The 

 mouth parts are strong and masticatory. The 

 insects go through complete metamorphosis. 



2. Order Hymenoptera, comprising the Ants, 

 Bees, Wasps, and Saw-flies. All the insects 

 included in this Order undergo complete meta- 

 morphosis. This group has generally four mem- 

 branous wings, which in the Bees and Wasps are 

 united during flight by a curious mechanism of 

 minute hooks that engage in a small groove at the 

 posterior edge of the front wings. The Hyme- 

 noptera are divided into two divisions, according 

 to the use of the ovipositor. Thus the Saw- 

 flies, Ichneumon Flies, Sirex or Wood Wasps, 

 and the Gall Flies, who all have the ovipositor 



