POLLINATION OF THE BRITISH PRIMULAS. 125. 
It should be remembered, however, that the size of a flower and the depth 
of its corolla-tube vary greatly in both forms, according to the vigour 
of the plant bearing it, which depends mainly on the favourable or 
unfavourable nature of the situation in which the plant happens to grow *. 
IV.— The Tongue-lengths of the Insects known to visit the Flowers. 
The average tongue-lengths of known insect visitors will be found 
appended (so far as I have been able to ascertain them) to the name of each 
insect mentioned in the lists given in the following Section. 
It will be recognized that, as in the case of the depths of corolla-tubes, the 
lengths of the tongues of visiting insects varies considerably in different 
individuals—at least 2-4 mm. in the case of the larger species of Bombus. 
There is, therefore, no fixed and absolute rule enabling one to say that any 
particular species of insect is or is not capable of reaching the nectar of 
(and, therefore, of pollinating) any particular species of Primula. 
Nevertheless, one is able to say this with practical certainty in the case of 
certain species of insects which have been seen to visit the flowers—namely, 
most of the larger humble-bees (some eight or ten species belonging to 
the genera Bombus and Anthophora), four species of butterfly, and one 
species of moth ; for all these have tongues ranging from 10 mm. to 21 mm. 
in length, and are thus able to reach the nectar of most of the flowers of all 
the three species. There are also a few smaller bees (some four or five in 
number, belonging to the genera Bombus and Apis), as well as at least two- 
species of bee-fly (Bombylius), which have tongues ranging from 7mm. to 
12mm. in length; and these are able to reach the nectar of some of the 
smaller flowers of probably all the three species. 
Further, one must bear in mind that, for a bee to reach the nectar of any 
Primula flower, it is not necessary for it to possess a tongue of the full length 
of the corolla-tube of that flower; for the heads of some bees are elongated 
and tapering, and thus may be inserted into the mouth of the corolla-tube 
* Thus, it would certainly be found that, in the Oxlip, the average size of both forms of 
flower on the Alps (where the plant grows in the open) is less than in flowers in this 
country (where the plant usually grows in woods and is more luxuriant on that account). 
+ For these, I have been obliged to rely almost wholly on the measurements given by 
Miiller and Knuth; for British writers have strangely avoided this subject. Thus, Saunders, 
who published an elaborate paper on the tongues of British bees (Journ. Linn. Soc., Zool. 
xxiii. pp. 410-432), does not give one single measurement. The matter, though important 
from the present point of view, is of little value to the systematist ; for, owing to the great 
variation in the sizes of individuals in many species of bees, both © and ¢ (this depending 
on the available food-supply), tongue-lengths also vary much, and are, consequently, of little 
value as specific characters. For advice on this point, I am indebted to the kindness of 
Dr. R. C. L. Perkins, F.R.S., Dr. James Waterston, F.E.S., and Mr. W. H. Tams, F.E.S. 
