10 DESCRIPTION OF PARTS. 



pleura and peripUura; the latter supports the halteres or poisers in dipterous in- 

 aeoto. Tlie thorax invariably supports six legs. The wings are sometimes wanting, when 

 ttie '» !««*«<* is said to he apterous : those with two wings are dipterous; and those with four, 

 tth^erous. The superior wings are provided witli an articulating socket, which is formed 

 in part by the organ called the scapula : they lie close upon the sternumj they constitute 

 the second principal portion of the raesothorax, lying one on each side in front. Another 

 distinct portion of the mesothorax is the mesosternum : it lies upon the under side, opposite 

 the mesonotum ; in its area the middle legs are articulated. The mesosternum is not equal- 

 ly developed in all orders of insects : it is most perfect in the Coleoptera and Orthoptera ; 

 the dorsal portion has received the n&me of metanotum, quadrangular in form, and emargi- 

 nate in front. 



The parts supported by the thorax are the wings and legs ; and, as has been observed 

 already, the latter are invariably six in number, and never wanting. The wings are variable 

 in number, in form and in substance ; and upon the variability in form and substance, the 

 orders are composed. Thus those with four wings are termed tetraplerous ; those with two, 

 dipterous; and those with none, apterous. The anterior pair in Tetraptera is attached to the 

 mesothorax : this pair is also sometimes called the superior pair, while the pair attached 

 to the metathorax is called the posterior or inferior pair. The pair which is wanting in the 

 Diptera, is the posterior pair. 



It has been said that the wings differ in form and substance. Nature, however, preserves 

 rigorously the symmetry of parts. In Coleoptera, the anterior wings differ from' the posterior 

 both in substance and form. The substance of the first is leathery, more or less rigid, and 

 horny : their form is various, but adapted to the dorsum or back of the insect ; while the 

 posterior are membranous, flexible and elastic, and capable of being folded beneath the 

 anterior wings. In another order of Tetraptera the wings are similar in form and substance, 

 and of equal size : these insects have received the name of JVeuroptera. Where, however, 

 in tctrapterous insects, the posterior wings dilfer in size, but preserve the form and sub- 

 stance of the anterior, the order is called Hymenoptera. Examples of Neuroptera are the 

 drftgon-flies ; of Hymenoptera, the common honey-bee, wasp, etc. In flight, the rigid an- 

 terior wing of the Coleoptera is of little use : it serves to protect the membranous wing 

 and abdomen when at rest. Hence it is that the flight of beetles is short, and sustained 

 evidently with much more labor than that of dragon-flies or honey-bees. The insects whose 

 wings are four, and covered with scales, are called from this last circumstance Lepidoptera. 



It is proper here to remark, that while there is a great dissimilarity between the anterior 

 wing of a beetle and that of a dragon-fly, yet we are not to suppose that the difference is 

 effected by a sudden leap : there is really a gradation in substance. We have insects whose 

 anterior wings are intermediate in this respect, both in substance and form. Thus the com- 

 mon grasshopper has an anterior wing which is between leathery and membranous, and is 



