180 Transactions of the Society. 



V. — Description of a New Dipterous Insect, Psamathiomya pectinata. 



By JuLiEN Deby, F.E.M.S. 



{Read 13th March, 1889.) 

 Plate IV. 



At the meeting of the Society held on the 9th of May last, I 

 exhibited slides of an interesting dipteron found by myself in abun- 

 dance during the latter days of last April, at Biarritz, in the South 

 of France. At the time I was not prepared to name or to describe 

 it, but having since come to the conclusion that it belongs to a new 

 genus and species, I now describe it in detail. 



Psamathiomya pectinata is a marine insect, living below water 

 during its early existence, the larvse feeding on Enteromorpha. The 

 adult escapes from the pupa case while the descending tide has laid 

 bare the algse-covered rocks; these small insects swarm at such 

 times, being especially active when the sun shines on them. The 

 males are more numerous than the females, and are also much more 

 rapid in their motions. I have often seen several males surrounding 

 one female, but I never caught any of these insects actually in copula, 

 though I frequently saw the males seize the heavy pregnant females 

 by the back of the head or neck by means of their formidable anal 

 forceps, and drag them forcibly along after them, stopping occa- 

 sionally, as if to rest, when the female would bend down her ovi- 

 positor and probing right and left with it, would, I believe, deposit 

 each time an egg among the green weeds or in some cranny of the 

 rocks below them. 



Both sexes have very rudimentary wings, quite useless as organs 

 of flight, so that these insects cannot possibly escape from the 

 rising tide, which on this coast is accompanied by heavy surf and 

 breakers. I presume therefore that the life of the imago does not 

 exceed the few hours during which the tide has receded. Several 

 specimens which I immersed in a phial of sea water were immediately 

 drowned. The insects being small have to be looked for with atten- 



EXPLANATION OF PLATE IV. 



Eig. 1. — Psamathiomya pectinata Deby, male 12/1. 



2. — Head seeu from above. 



3. — „ below. 



4. — Anal forceps of male. 



5. — Ovipositor seen laterally. 



6. — „ from above. The internal blades are figured too long. 



7. — Head and thorax from above. 



8. — Leg of male. 



9. — Terminal tarsal joint of male with its appendages 300/1. 

 10. — Wing and haltere of male, 60/1. 

 11. — „ „ female. 



Note. — The arrangement of the setse in some of the above figures, which were 

 executed during my absence from England, is not quite true to nature, so that 

 references to the text only must be relied upon in case of apparent discrepancies. 



