78 THE COCKROACH : 



(1) A pair passing from the posterior edge of the arms 

 of the medifurca to the stem of the postfurca ; (2) a pair 

 which diverge from the stem of the postfurca and proceed to 

 the fore part of the second abdominal sternum ; (3) a pair 

 passing from the posterior edge of the arms of the postfurca, 

 these are directed inwards and backwards, and are inserted into 

 the hinder part of the second abdominal sternum ; (4) a pair 

 already mentioned, which correspond in position and action to 

 the tergo-sternal muscles, and spring from the stem of the post- 

 furca, passing upwards and outwards to the sides of the first 

 abdominal tergum. 



The muscles attached to the arms of each furca pass to other 

 structures in or near the middle line of the body. The pull of 

 such muscles must alter the slope of the two steps in the 

 ventral floor of the thorax (p. 58, and fig. 3, p. 12). When the 

 furca is drawn forwards, the step is rendered vertical or even 

 inclined forward, the sterna being approximated ; while, on the 

 other hand, a backward pull brings the step into a horizontal 

 position, and separates the sterna. 



TERGAL MUSCLES OF THORAX. The longitudinal tergal muscles 

 are much reduced in width when compared with those of the 

 abdomen. Sets of obliquely placed muscles, which may be 

 called the lateral thoracic muscles, arise from near the middle of 

 each tergum, and converge to tendinous insertions on the fore 

 edge of each succeeding tergum, close to the lateral wall of the 

 body. 



The principal muscles of the legs are figured and named, and 

 their action can readily be inferred from the names assigned to 

 them. 



Insect Mechanics. 



The mechanics of Insect movements require exposition and 

 illustration far beyond what is possible in a book like this. 

 Even the elaborate dissections of Lyonnet and Straus-Durckheim 

 are not a sufficient basis for a thorough treatment of the sub- 

 ject, and until we possess many careful dissections, made by 

 anatomists who are bent upon mastering the action of the parts, 

 our views must needs be vague and of doubtful value. Zoologists 



