tJit Brilish Machilulse. 11 



It may be distinguished from the otlier British genus of 

 Machilidse, Prcumachilis, Grassi, by the presence of two pairs 

 of exsf^rtile vesicles on the second to the fifth abdominal 

 segments inclusive, Pnemacldlis having not more than one 

 pair of vesicles on any abdominal segment. 



Genus Petrobius, Leach (1809). 

 Petrobius vectensi's, sp. n. (Pis. I. & II.) 



Body-length 8 mm. (male). Feelers (incomplete) at least 

 as long as the body; tail-processes (incomplete) also at least 

 as long as the body. Feelers, cerci, and tail-process white- 

 ringed. Paired ocelli dumbbell-shaped, less than a transverse 

 diameter apart (PI. I. fig. 2). Mandible with blunt apex 

 (PL I. figs. 3 & 4 fl). Maxilla with lacinia slightly shorter 

 than galea (PI. I. figs. 6 & 6 a). Palp having its six elongate 

 segments with proportional lengths 6 : 8 : 10 : 11 : 11 : 10. 

 In the eighth alDdominal segment of tlie male the subcoxse 

 are produced into prominent rounded lobes (PI. II. fig. 1). 

 The ninth abdominal segment has the subcoxse produced into 

 small pointed processes (PI. II. fig. 2). Gonapophyses not 

 reaching quite to tips of subcoxae. Penis projecting to about 

 three-fourths length of stylet ; stylets with long, acute, apical 

 spines. In the eighth abdominal segment of female the 

 subcoxge are ))roduced into acute processes projecting to 

 about ^ length of stylet. Ovipositor of female nearly as long- 

 as the cerci, which are less than half the length of the 

 incomplete tail-process. 



Loc. (1) Shanklin, Isle of Wight ; collected by 'Sir. E. 

 Popple, Berkhamsted, Mav 1913. (2) St. Helen's, Isle of 

 Wight; collected by ^Ir. P. A. Buxton, Aug. 1912. 



Tiiis species agrees with P. h-evistylis. Carpenter (1913), 

 in having the mandible with blunt truncated apex, and 

 contrasts with P. maritimus, Leach (1809), the mandible of 

 which is acute. It agrees in many of its characters with the 

 Lutch insect which Oudemans designated Machi/is maritima 

 and described in his memoir (1886). Prof. G. H. Carpenter, 

 in a paper on the "Irish Species of Petrohius'' (1913), 

 referred to the latter insect, pointed out that it is certainly 

 not the true inaritinius of Leach, and suggested P. owc^ewa^isi 

 as an appropriate name for it. Oudemans, in his drawings, 

 fiiiures the lacinia of the maxilla much loncrer than the 

 galea; but in P. vectensis the lacinia is slightly shorter than 

 tlie galea, agreeing with P. brevisti/liSj Carpenter, while 

 the penultimate segment of the maxillary palp is not twice 



