Mr Lamb, Insect Oases 347 



Insect Oases. By C. G. Lamb, M.A. 



[Read 7 March 1921.] 



Cases of extremely limited distribution are familiar to all col- 

 lectors, the limitation being sometimes so great as to amount to 

 a single tree or a few square yards of ground, but it is nearly always 

 possible to correlate the distribution with the presence of the 

 necessary pabulum or with the environment. The establishment 

 of a satisfactory case of a persistent isolated colony requires not 

 only a careful search of the locality in respect to space, but obser- 

 vation over a considerable period of time in order to eliminate 

 possible secular disturbances. It happens that the author has 

 visited a particular locality at the same period of the year for 

 many years and has investigated it with much care, so that the 

 required time and space conditions may be taken to be well 

 satisfied. Further, the part of the district to be dealt with has the 

 additional advantage of being singularly homogeneous in its flora. 

 It consists of a tract of "towans" or sandy waste in the parish of 

 St Merryn, N. Cornwall, known as "Constantine Commons": this 

 waste is of fair extent and is characterised by great uniformity in 

 its flora which includes an exceptional number of the Boraginaceae, 

 Echiiim at times forming a perfectly astonishing spectacle: it also 

 bears the spotted hemlock, the henbane and the opium poppy in 

 fair plenty. The subsoil is clay, so that there is always permanent 

 water in parts, and it is intersected in places with ancient slate 

 slab walls bearing very old tamarisk bushes which indeed form 

 the only shelter against the gales. The district is full of archaeo- 

 logical interest and would repay investigation, and the neighbour- 

 hood has yielded several other insects of much interest, though 

 not exhibiting the locaHsed habit of those to be considered. In 

 this perfectly homogeneous area, and in places which careful in- 

 vestigation shows to be in no way different from the surroundings, 

 certain species of Diptera appear to inhabit quite definite oases or 

 islands. One may proceed to such a spot at the proper season 

 with the certainty that the insect will be found, and that in fair 

 or even great numbers, while the rest of the area may yield one 

 or two specimens at most, and that with extreme rarity. In spite 

 of much investigation the author has not been able to find any 

 circumstances whatever correlated to the distribution in the 

 examples given below: whether it be due to a "herd" instinct or 

 not is of course unanswerable: it is more probable that they are 

 cases of approaching extinction. It would be of much interest to 



