DESCRIPTION OF PARTS. 18 



is necessary tluit it should be well supplied with air or oxygen : hence each fegmtnt has 

 a pair of spiracles, which are capable of inhaling a large amount of this essential element. 

 The external orifices are often protected by hairs, which shut over them, and thus serve to 

 exclude fijreign matters, while the air permeates freely into the interior of the body. 



Besides the digestive organs, and a portion of the respiratory apparatus, the abdomen 

 c>nt;iins the genital organs, the piercer and sting of females. The piercer is a flexible jointed 

 tube, and is iisi-d to i)imcture trees or other bodies for the purpose of depositing the eggs. 

 It varies in length and I'urm, and is titled to {K-rform its office according to the instinct of 

 the animal to which it belongs. 



I have now described, in as few words as possible, the most essential external parts of 

 insects ; those parts which are particularly employed in the description of genera and 

 species, as well as those used in general classification. From these parts the sttident will 

 be able to form a correct notion of the extent of this class of animals ; but there are yet 

 other portions, occupying the interior of the insect's Ixxly, which are important for us to 

 study, inasmuch as they will yieUl information in regard to the economy of this interesting 

 class which will be of essential service. I refer to the diiresflvc, circulator]/, and nervous 

 systems. If the external fjrms of insects .ore curious and interesting, the student will find 

 that the apparatus lor carrying on the foregoing functions is still more so. There is espe- 

 cially one curious fact particularly worthy of notice in relation to the digestive apparatus : 

 it is this, that the secretory organs are reduced to the simplest form, that of tubes ; from 

 which we learn that nature requires no special form of instrument for the performance of 

 a given function. 



All the essentials of a digestive apparatus, which belong to the higher classes of animal.i, 

 are found in the insect. Some interesting differences, it is true, exist ; still wlien it is con- 

 sidered that the food of insects does not really differ from that of other animals, we may 

 of cjiirse expect to find the digestive organs essentially the same in kind. As insects subsist 

 upon various kinds of food, some upon vegetables, others upon the elaborated juices of 

 animals or upon flesh, so we find similar variations in the form and proportion of the organs 

 as exist among the higher animals. The vegetable feeder has a larger and more capacious 

 digestive apparatus than the carnivorous insect, nature always adapting her means to the 

 end. 



The function of digestion in insects, as in all other animals, is performed through the 

 instrumentality of an alimentary canal. Our first object will be to describe this apparatus. 

 The function itself involves the existence of two kinds of apparatus : the parts through 

 which the food must pass; and the parts which siii)ply the special fluids essential to effect 

 oert;iin changes in the food before it can become aliment or nutrient matter. Beginning 

 with the description of the first kind of apparatus, namely, the organ or organs through 

 which the OkkI passes during the process of digestion, we may regard this apparatus as a 



