ASTACID.E. 25 



genera Lupa and Callinectes there are not rarely females witli a very 

 narrow and acute postabdonien. These it is very easy to separate from 

 the ordinary females, with large and circular postabdonien. Professor 

 L. Agassiz informs me that he has satislied himself, by an anatomical 

 examination of living specimens, that these females ai'c sterile. I have 

 found similar females with a narrower triangular abdomen in some 

 other genera of Brachi/ura. 



I am indebted to Mr. Alexander Agassiz for the information that F. 

 MUUer, Fiier Darwin, 18G4, has described two forms of the male in 

 Orchcstia Darwimi and in Tanais dubius. He remarks that when found 

 upon the shore the form of the second pair of gnathopoda varies fi-om 

 that of specimens found at a distance iidand, where it lives under moiddy 

 leaves in loose earth. In 0. Danviuii, intermediate forms between the 

 males with large and those with small hands are not to be detected, but 

 in two other sjiecies, 0. tnciirauna and 0. tucuratimja, the shape of the 

 antenna^ and of the hands changes even in the full-ffrown males. 



The supposition that the first-form males only in Cantharus possess 

 large hands for burrowing purposes is to be rejected, as the females also 

 have the same burrowing habits. 



The existence of two different forms of males in Camhants is very 

 important in the description of the species, and the ftict that these 

 forms are not recognized by all preceding authors may explain some 

 erroneous determinations in their works. 



DimorpJusm in Insects. — The discovery of a dimorphism in the Crus- 

 tacea is all the more interestina;, since as yet in the whole animal king- 

 dom dimorphism was kno\vn only in the insects. There are many facts 

 and communications scattered through entomological literature, of which 

 a general review is very desirable. An anatomical examination of these 

 dimorphic forms is still wanting, only the external differences having 

 been thus ftir marked. 



The dimorphism seems to be represented in two different ways ; a 

 difference only in the colors (dichroic forms of Brauer), or a difference 

 in size and shape, and mostly in the female. I should remark that 

 dimorphism, as observed in insects, occurs only in one sex of the same 

 species, and mostly in the female. Perhaps in the ants and in the 

 white ants — it seems more natural to range all the socially living 

 insects, viz. the ants bees, wasps, and white ants under the same law 

 — a dimorphism is to be found in l)oth sexes. 



Dimorphism consisting in different colors w\as long since observed, 

 especially in Lepidoptera, in the hind wings of many Orthoptera, and in 

 the females of Agrion. In the latter genus the well-known orange- 

 colored females are probably sterile. 



Dimorphism with difference in shape and size is also often observed. 

 A very common case is the difference in the development of the wings. 



4 



