DESCRIPTION OF PARTS. 18 



is necessary that it should be well supplied with air or oxygen : hence each segment 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 ovet them, and thus serve to 

 exclude foreign 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 

 ontains the genital organs, the piercer and sting of females. The piercer is a flexible jointed 

 tube, and is used to puncture trees or other bodies for the pm-pose of depositing the eggs. 

 It varies in length and form, and is fitted to perform 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 student 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 body, which are important for us to 

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

 class which will be of essential service. I refer to the digestive, circulatory, and nervous 

 systems. If the external forms of insects are curious and interesting, the student will find 

 that the apparatus for 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 animals, 

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

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

 of course exjiect 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. OiU" 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 supply the special fluids essential to effect 

 certain 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 food passes during the process of digestion, we may regard this apparatus as a 



