162 Mr. J. O. Westwood 07i the Earwig. 



the 7th spiracle gave rise to a much more developed and ramified 

 trachea than in the female, and which extended as in that sex to 

 the caudal forceps. 



Thus in this insect there are ten consecutive segments, including 

 the three thoracic ones, each of vi'hich bears a pair of spiracles. 



The structure of the thorax in this insect is also worthy of obser- 

 vation. The insect is a very good walker and its legs are of equal 

 size, it is essential therefore that the sternum of each of the three 

 thoracic segments should be nearly equally developed, and such is 

 the case. The insect is also a flyer, but its posterior wings only are 

 capable of assisting in aerial action, its minute tegmina being appa- 

 rently incapable of rendering it any assistance. The tergum of the 

 mesothorax is therefore very short, whilst that of the metathorax is 

 large and nearly square ; it is divided into two parts in a curious man- 

 ner, the interior angles of the lower wings being produced internally 

 in a narrow band until they meet in the middle of the back, and are 

 extended backwards in a point. I know no other instance in which 

 this character is to be observed. In the Diptera indeed the inner 

 basal angle of the wings is internally produced behind at the sides, 

 forming the alulae ; but in that order it is the anterior wings which 

 are thus dilated, whilst in the earwig it is the posterior wings, a ma- 

 terial point, and one which tends to show that where one organ in 

 one group assumes the offices of another organ in a distinct group, 

 the peculiarities of the former may be observed in the latter. 



DESCRIPTION OF THE FIGURES. 



Fig. 1. The three thoracic and two abdominal segments seen laterally, the limbs 

 removed. 



Fig. 2. The meso- and meta-thorax seen from above, the dorsal metathoracic spi- 

 racle (a) on one side being observed by the removal of the extremity of 

 the anal lobe of the wing (6). 



Fig. 3. The mesothorax alone seen laterally. 



Fig. 4. The metathorax alone seen laterally. 



Fig. 5. The mesothoracic spiracle, with the base of the several tracheae to which 

 it internally gives rise. 



Fig. 6. The abdomen of the male seen laterally, the segments numbered 1 to 9, 

 and the spiracles lettered a to g. 



Fig. 7. The three terminal segments of the abdomen of the male opened laterally, 

 showing the development of the trachea arising from the seventh spiracle, 

 the central tracheae being truncated. 



Fig, 8. Representing the part of this trachea which was removed, and which ex- 

 tended to the caudal forceps. 



Fig. 9. The abdomen of the female seen laterally and distended : a, the anus ; 

 b, one of the anal triangular plates. 



