8 THE INSECT WORLD. 



by a resisting medium, the legs on both sides are moved at the 

 same time. 



The act of jumping is principally performed by the hind legs. 

 Insects which jump have these legs very largely developed, as in 

 Fig. 9. When about to jump they bring the tibia into contact 

 with the thigh, which is often furnished with a groove to receive it, 

 having on each side a row of spines. The leg then suddenly 

 straightens like a spring, and the foot being placed firmly on the 

 ground, sends the insect into the air, and at the same time propels 

 forward. The jump is greater in proportion as the leg is longer. 



To treat here in a general manner of the wings of insects 

 would be useless. We shall refer to them at length in their proper 

 place, when treating of the various types of winged insects. 



In the perfect insect the abdomen does not carry either the wings 

 or the legs. It is formed of nine segments, which are without ap- 

 pendages, with the exception of the posterior ones, which often 

 carry small organs differing much in form and function. These are 

 saws, probes, forceps, stings, augers, &c. We shall consider these 

 different organs in their proper places. 



With vertebrate animals, which have an interior skeleton suited 

 to furnish points of resistance for their various movements, the skin 

 is a more or less soft covering, uniformly diffiised over the exterior 

 of the body, and intended only to protect it against external injury. 

 In insects the points of resistance are changed from the interior to 

 the exterior. The skin is altered by Nature to fit it to this purpose. 

 It is hard, and presents between the segments only membranous 

 intervals, which allow the hard parts to move in all directions. 



We are examining a perfect insect ; we have glanced at its skele- 

 ton, and the different appendages which spring from it. The prin- 

 cipal organs which are contained in the body remain to be examined. 



We will first study the digestive apparatus. This apparatus con- 

 sists of a lengthened tubular organ, swollen at certain points, forming 

 more or less numerous convolutions, and provided with two distinct 

 orifices. This alimentary canal is always situated in the median line 

 of the body, traverses its whole length, and is at first surrounded by, 

 and then passes above, the nervous ganglia.* 



In its most complicated form the alimentary canal is composed 

 of an (esophagus^ or gullet, of a crop, of a gizzard, of a chylific ven- 

 tricle or stomach, a small intestine, a large intestine, divers appendages, 

 salivary, biliary, and urinary glands. The oesophagus is often not 



* Gangliojt — a mass, literally a knot, of nervous matter. 



