THE DISTRIBUTION, RARITY, OR ABUNDANCE OF VARIOUS SPECIES
There are few points about which we know less than the causes of distribution and rarity, although there are certain tolerably well recognized laws which govern the occurrence of some species in certain localities. What I mean is that marshy spots, say salt marshes for instance, attract certain beetles and bugs which are never found except in such places; certain kinds of flowers attract bees which never appear to visit any others, but these localities and kinds of flowers occur often at great distances from each other, and why—given a certain flower you probably find a certain bee peculiar to it; or given a certain kind of marsh you probably find a certain beetle, although the localities may be hundreds of miles apart—I think still awaits explanation. I will give an example with which I am personally well acquainted.
There is a rare little bee (Macropis labiata) which at one time was looked upon as an extreme rarity, having only occurred three or four times in this country. Mr. F. Enoch, comparatively lately, took a fair number on the flowers of the greater loose-strife (Lysimachia vulgaris) along the canal at Woking; now that its food-plant is known, it has occurred in several other places in numbers, and no doubt wherever the Lysimachia is abundant Macropis will probably occur, but how the little creature has been distributed over the places where this plant occurs, which are often far distant from each other, seems to me to be an unsolved problem. Then there is another puzzling point, and that is the extreme rarity of certain insects. No doubt in many cases this is due to ignorance of their habits, as it has frequently happened that species once considered of great rarity have occurred in abundance when their habits have been discovered, as in the case of Macropis, but there are some cases which do not seem to be explainable in this way. I will again give an example which has been specially under my own observation. Dufourea vulgaris, a little black bee,
which certainly might not be recognized from its outward appearance, as there are many which very closely resemble it, is still one of our greatest rarities, only three British examples having been recorded. The first was taken by Sir Sidney Saunders at Chewton, Hants, on the twelfth of August, 1879; this was a male; the second, a female, was taken by Mr. T. R. Billups at Woking, on the first of August, 1881; and the third by myself at Chobham (about four miles from Woking) on the first of August, 1891. I believe in all cases these were taken on yellow composite flowers. The flight and behaviour of the male I caught were so peculiar, as it wriggled itself into the flower, that I knew at once I had caught a rarity, and remarked to my companions that I believed I had got a Dufourea. I also hazarded the remark that it was "ten years since it had been taken." When I got home and looked up the former record it was ten years to a day. Now there are few places in England that have been better worked for the bee tribe than the Woking, Chobham, and Weybridge neighbourhood; it has been worked by experienced men who would see a difference
in the flight of an insect directly. The late Mr. F. Smith, in his day our leading authority, the Rev. F. D. Morice, than whom no one has probably worked the neighbourhood more thoroughly, Mr. T. R. Billups, Mr. E. B. Nevinson, and the late Mr. A. Beaumont, have all been over the ground again and again, and yet only these two Dufoureas! and these taken four miles apart. Here again is a problem which is very perplexing! What part in nature does this little rarity play? No doubt like everything else it has its duties, and its corner to fill, but beyond that one can suggest nothing.
Other bees are often exceedingly abundant in one season and very rare the next, or they will entirely desert a locality where they have been abundant, and move somewhere else—the occasional scarceness is due probably to continued wet weather, which often appears to kill the larvæ. Cold winters seem to have no injurious effect, although at one time they were thought to determine the scarcity or otherwise of the bees of the following summer. It has, I think, been clearly shown that larvæ can stand almost any amount of cold, although they succumb to
the effects of mildew produced by wet, but there is often no apparent reason why a well established colony should migrate to quite new pastures. Sometimes the proximity of new buildings or the digging up of ground may disturb them, but I know of colonies that have gone from where I knew them a comparatively few years ago, and where I can detect no change likely to have affected them. On the other hand there are colonies which one has known all one's life and which still go on as strongly or more strongly than ever—the case quoted under Anthophora, p. [63], shows what persistence there can be in some.