10 Miss Anna J. Reilly on 



II. — JSotes on the British Machilidae, with Descriptions oj 

 Two new Species. By Anna J. Reilly, A.R.C.Sc.L, 

 Royal College of Science, Dublin. 



[Plates I.-V.] 



In Lubbock's 'Monograph of the Collembola andThysanura' 

 (1873) there are descriptions of Machilis polypoda and Ma- 

 chilis maritima, only two species of the Machilidae being- 

 recognized in Great Britain at that period. Silvestri (1904) 

 justified the establishment of the genus Petrobhis by Leach 

 (1809) for M. maritima by showing that Machilis has paired 

 processes (gonapophyses or paramera) on both the eighth and 

 ninth abdominal segments in the male, while Petrobhis has 

 these structures on the ninth segment only. Verhoeff (1910) 

 established the genus Halomachilis, characterizing it by the 

 following structural details : — apex of the mandible un- 

 toothed, feelers without scales except on the two basal seg- 

 ments, tip of labial palp having many flattened sensory 

 spines. These details prove this genus to be identical with 

 Petrobius. 



Many species of Petrobius have since been recorded ; 

 Otidemans (1886) described a Dutch insect which he called 

 Machilis maritima, and which agrees in many respects with 

 the new species Petrobius vectensis described in this paper. 

 I am indebted to Mr. R. S. Bagnall and Mr. E. Popple, 

 Berkhamsted, for these specimens of P. vectensis ; they were 

 collected near Shanklin, Isle of Wight. I am also indebted 

 to Mr. P. A. Buxton, Trinity College, Cambridge, for some 

 specimens of this species which were collected by him at 

 St. Helen's, Isle of Wight. 



The second species described falls into a new genus of the 

 Machilidae — Petromachiiis. The specimens were collected 

 in August 1911 at Wasdale Head, Cumberland, by Mr. J. W. 

 Shoebotham, who sent them to Prof. G. 11. Carpenter, Royal 

 College of Science, Dublin, for description. Prof. Carpenter 

 kindly asked me to describe these and the Isle of Wight 

 insects mentioned above, giving me some help and instruction 

 in methods of procedure. 



The genus Petromachiiis is intermediate between Machilis 

 and Petrobius, agreeing with Machilis in having paired 

 processes on both the eighth and ninth abdominal segments 

 in the male, and with Petrobius in having no scales on the 

 antennae except on the two basal segments, while it is inter- 

 mediate between the two genera with regard to the mandible. 



