STRUCTURE OF INSECTS. 15 



having a pair of legs attached to it. The first ring of the thorax 

 (d) never has wings attached to it, and is always visible, while 

 the succeeding rings are commonly covered above by these 

 organs. When there are four wings, which is almost always the 

 case, those of the first pair are attached to the second ring of the 

 thorax (f), and are covered by the next pair, which are inserted 

 into the sides of the third thora'cic ring (i). When there is only 

 one pair of wings (as in the common fly), they are attached to 

 the second ring of the thorax (/). 



The first ring of the thorax (d) is called the prothorax (from the Greek, 

 pro, before, and thorax, shield, or chest); the second ring (/), mesothorax 

 (from the Greek, mesos, the middle, and thorax') ; and the third (t) the meta- 

 thorax (from the Greek, meta, between, and thorax"). 



These three rings are closely and solidly united into one piece, and 

 constitute the trunk, the inferior surface of which is styled the peclus ; that 

 portion of it which corresponds to the prothorax, is called ante-pectus (from 

 the Latin, ante, before, and peclus, breast) ; that portion which corresponds 

 to the mesothorax, is called medio-pectus (from the Latin, medius, the 

 middle, and pectus, breast) ; and the part corresponding to the metathorax, 

 is named post-pcctus (from the Latin, post, behind, and pectus, breast). The 

 middle line of the inferior surface of the trunk is termed the sternum, and 

 is divided into three parts ; the ante-sternum, medio*sternum, and post- 

 sternum. 



23. In all true insects, or, as they are also denominated, hexa- 

 pods (from the Greek, exa, six, and pous, foot having six feet), 

 the abdomen is very distinct from the thorax, and has no ex- 

 tremities, neither feet nor wings, attached to it : it is composed of 

 a certain number of rings, and we often find at its termination, 

 near the anus, various appendages, such as stings or borers. The 

 last rings or an'nuli of the abdomen, in several females, form a 

 retractile or always projecting ovipositor, of a more or less com- 

 plicated structure, which acts as an auger. 



24. The legs of insects, which are solid tubes containing the 

 muscles by which they are moved, are always six in number ; 

 there are never fewer than six, and if in some instances we see 

 but four at first (as in certain butterflies, Papilio), we shall find 

 on close examination that two of these organs are not developed, 

 but are concealed under the hair. 



25. Sometimes the legs are formod solely for walking ; some- 

 times they are elongatecLand fitted for leaping, or they are spread 

 out so as to constitute fins for swimming ; and, again, they are 

 modified in such a manner as to form oi^ans of prehension. 



23. What extremities are attached to the abdomen ? 



24. What is the invariable number of legs in insects ? Where are the 

 muscles placed which move the legs ? 



25. Are the legs of all insects alike ? What are the uses to which tney 

 are applied ? 



