MD 
Miscellaneous. 71 
An Instance of Sexual Colour-variation in Crustacea. 
By H. W. Conn*. 
Differences in the colour of the two sexes among Crustacea are of 
very rare occurrence. Darwin in ‘The Descent of Man,’ chap. ix., 
refers to this fact and says he is acquainted with but two instances 
of this peculiarity. One in the case of Squilla stylifera, and a 
second in a species of Gelasimus, or fiddler crab, described by Fritz 
Miiller as occurring in Brazil. A third and very striking instance 
is found in Callinectes (Neptunus) hastata, the common edible crab 
of our southern coast. There are a number of differences in the shape 
of the two sexes, but besides these they present a marked difference in 
colour. This colour-variation is confined to the first pair of thoracic 
appendages, the pair bearing the large chele. These appendages are 
of a yellowish brown on the upper surface, a whitish yellow on the 
outside, and of a brilliant blue on the inside and particularly at those 
parts which are protected from the light when the appendage is 
folded. It would seem therefore that this blue coloration was en- 
hanced by not being exposed to light. The colour of different 
individuals is tolerably constant and uniform, 
Between the colours of the male and female appendages considerable 
differences are discernible. The most noticeable difference is that 
the male appendage appears remarkably blue when compared with 
the female. ‘This is due partly to the fact that the amount of blue 
surface in the male is much greater than in the female, and partly 
to the fact that the blue colour is of a much more brilliant hue. The 
blue colour in the male extends nearly to the tips of the two fingers 
of the chele, both the finger-like process of the propodite and the 
dactylopodite being largely coloured blue. The very tips are, how- 
eyer, of a brilliant purple. In the female these parts are of an 
orange hue, with not a trace of blue about them. ‘The tips are 
also coloured purple, but not so brilliant a purple as is found in the 
male. In the male the blue colour extends partly upon the outer 
surface. In the female it is confined to the inner surface and only 
extends to the base of the dactylopodite. The outer surface of the 
dactylopodite and of the finger-like process of the propodite are in 
the male white, while in the female they are reddish orange. Upon 
the male appendage there is no orange colour as a rule. 
These differences in colour are in all cases very marked, and will 
always serve to distinguish a male from a female appendage. No 
colour-differences are seen in any part of the crab except upon the 
first pair of appendages ; and it is interesting to note that this sexual 
difference does not make its appearance till the crab reaches ma- 
turity. The chele of immature males and females cannot be dis- 
tinguished from each other. Fritz Miiller says that the same is 
true of the Gelasimus observed by him. On the other hand, con- 
sidering the habits of Crustacea, these sexual differences can hardly 
be considered as the results of sexual selection.—Johns Hopkins 
University Circulars, Noy. 1883. 
* Note from the ‘ Chesapeake Zoological Laboratory,’ 1883. 
