Mr. J. 0. Westwood on the Earwig. 161 



thorax, and found that what I had regarded as mesothoracic and 

 metathoracic spiracles were really such, giving rise to numerous 

 tracheae. 



Thus each of the three thoracic segments is proved to be furnished 

 with a pair of spiracles ; and thus the argument of Latreille and 

 Audouin, that the terminal part of the hyraenopterous thorax must 

 in reality be the basal segment of the abdomen because it is fur- 

 nished with a spiracle, " caractfere qui ne permet pas de confondre 

 le segment avec le metathorax propre, puisque celui-ci en est de- 

 pourvu," is found to be unstable, at least if we may be permitted 

 to judge from the analogy offered in this instance. 



By considerably distending the abdomen of a female earwig the 

 spiracles are brought into view. They are, as M. L. Dufour ob- 

 serves, " d'une petitesse microscopique," and it is only by holding 

 the insect in certain positions that they can be observed at all even 

 with a lens of high power. They are placed at the frontal angles 

 of the recurved sides of the ventral plates, the angle itself being 

 slightly excised. The first 6 segments are thus furnished, and a 

 7th pair is observed at the extremity of the first of the two rudi- 

 mental joints, that is the seventh abdominal segment. There is 

 none however on the side of the second rudimental joint (the 8th 

 abdominal segment), nor can any be seen upon the terminal or 9th 

 segment. For the purpose however of ascertaining beyond a ques- 

 tion whether this 7th spiracle served for the supply of air to the 

 8th and 9th segments, I examined the internal structure of the ex- 

 tremity of the body, and by that means clearly perceived that this 

 7th pair of spiracles gave rise to a large trachea running towards the 

 head, and another slightly ramified one which extended not only over 

 the 8th and 9th segments but also ran into the caudal forceps. 



Now from the rudimental structure of the 7 th and 8th abdominal 

 segments in the female it is not surprising that the 7th spiracle 

 should extend to the extremity of the body, because as the 7 th, 8th, 

 and 9th segments are soldered together they are to be regarded as a 

 single joint, and consequently as furnished only with a single pair of 

 spiracles : but in the male the case is different ; the 7th, 8th, and 

 9th segments are here equally developed, and each has its separate 

 movements. I was therefore anxious to ascertain whether the 8th 

 and 9th segments of the abdomen of the male were furnished with 

 spiracles as well as the 7 th, because if such were the case we might 

 be led to expect the rudiments of such additional spiracles ought to 

 be found in the female. On the most careful examination, however, 

 I could find no external trace in the male of spiracles after the 7th 

 abdominal segment, and upon dissecting this sex I ascertained that 



