4 INSECTS AT HOME. 



finding them, for the real difficulty is to dissect any part of 

 the body without finding them. The largest of these tubes 

 are those which run along the sides of the insect, and are con- 

 nected with the oval openings along the sides, which are pos- 

 sessed by every insect. These openings are called spiracles, 

 from the Latin word spiro, because through them the insect 

 breathes. Any insect or caterpillar will furnish the trachea-, 

 but the larger the better. They should be severed from th*' 

 body by a pair of fine scissors, then taken out with a pair of 

 forceps, and laid on a glass slide. I have now before me a 

 preparation of the tracheae of a silkworm which I made twenty- 

 two years ago, and it is not the least damaged by keeping. 



These trachese afford a most important characteristic of the 

 insects, inasmuch as the Crustacea do not possess them at all, 

 and the Arachnida generally, though not always, breathe by 

 means of internal air-sacs. 



Next, the creature must be divided into three distinct portions. 

 This is the signification of the title Insect, which is derived 

 from two Latin words, signifying cut-into, while the familiar 

 Greek name of Entoma (from which the word entomology is 

 formed) has precisely the same signification. This is, perhaps, 

 the most important of all the characteristics, as in the Crustacea 

 and Arachnida the head is merged into the thorax, so that they 

 are divided into two portions instead of three ; while in the 

 Myriapoda and Annelida there is no distinct thorax, and some- 

 times scarcely a distinct head. 



Next we come to the transformations which insects have to 

 undergo before they reach their perfect or adult state. All 

 animals really undergo a course of transformation, but in the 

 insect they take four very distinct forms ; namely, the Egg, 

 the Larva (i.e. caterpillar or grub), the Pupa (or chrysalis), and 

 the Imago, or perfect insect. Any of my readers who have 

 had silkworms will be practically acquainted with this fact, 

 and will also know that the larva changes its skin, or moults, 

 several times before it assumes the pupal form. The reason 

 for this casting of skin is evident. The larva, like the perfect 

 insect, is made of a series of flattened rings, or rather, of a 

 double series of half rings, connected along the sides ly an 

 elastic membrane, so as to permit the creature to breathe and 

 oat. 



