MOUTH-PARTS OF LEPISMA. 129 



three thoracic segments are of nearly equal size, and the eight 

 abdominal segments are also of similar size. The tracheae 

 are well developed, and may be readily seen in the legs. The 

 end of the rather long and weak abdomen is propped up by 

 two or three pairs of bristles, which are simple, not jointed, 

 but moving freely at their insertion ; thus they take the place 

 of legs, and remind one of the abdominal legs of the Myri- 

 opods ; and we shall see in certain other genera (Machilis and 

 Campodea) of the Bristle-tails that there are actually two- 

 jointed bristles arranged in pairs along the abdomen. They 

 may probably be directly compared with the abdominal legs of 

 Myriopods. Further study, however, of the homologies of these 

 peculiar appendages, and especially a knowledge of the embry- 

 *ological development of Lepisma and Machilis, is needed before 

 this interesting point can be definitely settled. The three many 

 jointed anal stylets may, however, be directly compared with 

 the similar appendages of Perla and Ephemera. The mode of 

 insertion of the antennae of this family is much like that of the 

 Myriopods, the front of the head being flattened, and concealing 

 the base of the antennae, as in the Centipedes and Pauropus. 

 Indeed, the head of any Thysanurous insect seen from above, 

 bears a general resemblance in some of its features to that of 

 the Centipede and its allies. So in a less degree does the head 

 of the larvae of certain Neuroptera and Coleoptera. The eyes 

 are compound, the single facets forming a sort of heap. The 

 clypeus and labrum, or upper lip, is, in all the Thysanura, car- 

 ried far down on the under side of the head, the clypeus being 

 almost obsolete in the PodurJUae, this being one of the most 

 essential characters of that family. Indeed, it is somewhat 

 singular that these and other important characteristics of this 

 group have been almost entirely passed over by authors, who 

 have consequently separated these insects from other groups on 

 what appear to the writer as comparatively slight and inconsid- 

 erable characters. The mouth-parts of the Lepismatidse (espe- 

 cially the thermophilous Lepisma, which we now describe) are 

 most readily compared with those of the larva of Perla. The 

 rather large, stout mandibles are concealed at their tips, under 

 the upper lip, which moves freely up and down when the crea- 

 ture opens its mouth. The mandible is about one- third as broad 

 as long, armed with three sharp teeth on the outer edge, and 

 with a broad cutting edge within, and still further inwards a lot 



