POLLINATION OF THE BRITISH PRIMULAS. 123 
has observed Zlomalium florale * ; and in North Wales, Mr. Dallman has 
found over a dozen other species in the flowers at night f. 
Further, Mr. Dallman has shown f that, at night, Primroses are visited by 
various spiders, earwigs, slugs, snails, and wood-lice. 
Yet other, and still more minute, creatures which frequent the flowers 
of both Primrose and Oxlip are Thrips—very small, slender, elongated, 
blackish, partially-winged insects, scarcely one millimetre in length, 
belonging to the genus Tæniothrips of the order Thysanoptera. These are 
often found in abundance on the leaves and petals of plants, especially roses, 
carnations, and chrysanthemums. They are probably all sap-suckers, and 
often do much injury to the plants on which they live. To some extent, 
perhaps, they also consume pollen. In Primula flowers, I have seen them 
often, but always in very small numbers—generally a single one or a pair. 
Darwin f, the Rev. E. Bell $, and Mr. Dallman f all speak, however, of having 
seen Thrips abundant in Primroses. The latter says they appear “ to occur 
wherever the Primrose is met with." Possibly Thrips enter the flowers 
largely for shelter, though one species (T. primulæ) seems to be associated 
sufficiently closely with the genus Primula to have been named after it. 
IIL— The Depths of the Corolla-tubes of the Flowers. 
Before considering what bearing the foregoing observations have upon the 
problem in hand, it is necessary to ascertain the average depths of the deep 
and narrow corolla-tubes of the flowers of Primula. This matter has a very 
intimate bearing on the problem ; for upon the depths of the corolla-tubes 
naturally depends the ability or inability of visiting insects to reach the 
nectar and to pollinate the flowers. 
In all the three species, the depth of the corolla-tube differs in the two 
forms of flower ; for the tubes of short-styled flowers are slightly deeper than 
those of long-styled flowers. This fact has been overlooked entirely by 
British botanists, even by Darwin ; but several Continental botanists have 
recorded their observations on the point. I summarize their results 
separately for each species :— 
Primrose.—The most satisfactory measurements are those of Cobelli, made in the 
Tyrol ||, which show that the average depth of ten short-styled flowers was 19:1 mm.: 
that of ten long-styled flowers, 17:7 mm.—a difference of 1:6 mm. 
My own measurements have been made in Essex. In April 1916, I found that 
six short-styled and six long-styled flowers each averaged 16:1 mm. ; but the number 
* See ante, p. 119. 
T Journ. of Botany, lix. (1921) pp. 320-322. 
t ‘Forms of Flowers,’ pp. 25, 37, 49, &c. 
$ * Primrose and Darwinism,’ pp. 33-26, &c. 
|| Verhandl. zool.-bot. Gesellsch. Wien, xlii. (1892), p. 74. 
