28 AN INTRODUCTION TO ENTOMOLOGY. 



a large part of the muscular system will be exposed to view. Plate 

 I. represents the thorax and abdomen of a larva which has been pre- 

 pared in this way. In these figures the band-like structures repre- 

 sented as lining the body-wall are muscles. And the number is much 

 greater than shown here ; for between these muscles and the body- 

 wall there are in most places several layers of diagonal muscles. 



The muscular system is composed of an immense number of dis- 

 tinct, isolated, straight fibres, which are always free (i.e., not inclosed 

 in tendinous sheaths as with Vertebrates). As a rule, the muscles 

 that move the segments of the body are not furnished with tendons 

 (Plate I.) ; while those that move the appendages are thus united at 

 the distal end (Fig. 38). In appearance the muscles are either 



Fig. 38. — Leg of May-beetle. (After Straus-Durckheim.) 



colorless and transparent, or yellowish-white ; and of a soft, almost 

 gelatinous consistence. When properly treated with histological 

 reagents, and examined with a microscope of moderately high 

 power, they present numerous transverse striations, like the volun- 

 tary muscles of Vertebrates. 



The Alimentary Cana/. — In the ideal figure given on page 25, the 

 alimentary canal is represented as a straight tube extending from 

 one end of the body to the other. In the larva of some insects there 

 is an approach to this degree of simplicity. But usually the tube is 

 longer than the body, and is consequently more or less convoluted. 

 Moreover, it is not of uniform structure, but, as in the higher animals, 

 different parts are adapted to different functions. Names have 

 been applied to these special parts similar to those used to desig- 

 nate the analogous parts in higher animals. These are as follows : 



There is within the head a portion of the alimentary canal that 

 is usually more or less enlarged ; this is the pharynx. It has been 

 shown recently * that in some sucking insects the pharynx is fur- 

 nished with powerful muscles, by which it can be distended, and 

 that it is doubtless the pumping organ, by which these insects 



* Edward Burgess, Contributions to the Anatomy of the Milkweed Butterfly (Me- 

 moirs of the Boston Society of Natural History, 1880). 



George Dimmock, The Anatomy of the Mouth-parts and Sucking Apparatus of some 

 Diptera (Boston, 1881). 



