18 



RESPIRATORY SYSTEM. 



Insects do not breathe through the mouth as mamalia, 

 birds, and other tribe*. They have a peculiar respiratory 

 system of their own. If we examine the caterpillar, for 

 instance, of the Privet Hawk Moth, Sphinx Ligustri, we 

 shall find there are nine little yellow oval discs on each 

 side called the spiracles, and which have already been 

 pointed out to you when speaking of the thirteen segments. 

 These little discs have an opening in the centre ; this 

 opening is surrounded and acted upon by a circular muscle 

 called a sphincter muscle. If I again refer to our lip, you 

 will have an illustration of the action of this muscle : upon 

 the least approach of a smile, it begins to relax ; if the 

 smile turn to a laugh, a visible distortion takes place, and 

 the mouth then opens ; but should screaming hysterics 

 follow, this poor circular muscle is then stretched to its 

 fullest extent, and it is wonderful how it ever comes again 

 to its engaging proportion. A sphincter muscle, then, 

 guards each orifice ; it dilates and closes it, allowing air to 

 pass in. These orifices or spiracles communicate by short 

 tubes with a longitudinal canal or pipe, one on each side of 

 the larva. These tubes send ofif numerous branches, which 

 divide and subdivide, allowing air to pass freely into every 

 part and every organ of the insect system. To this I shall 

 have again to allude presently. What I wish you to com- 

 prehend is, that by the universal ramifications of these air- 

 tubes or tracheae, as they are called, the juices or blood of 

 the creature is acted upon and rendered fit for the purposes 

 of life, as the blood of our system is afiected by the air 

 passing into our lungs, and there altered in its character to 

 perform a similar office. 



REPKODUCTITE SYSTEM. 



We shall not stop to enquire into this subject ; but I may 

 mention that the eggs of the future brood are to be dis- 

 covered in the larva. We shall now enquire into the 

 important and interesting physiology of the nervou* 

 system. 



