DIAPTOMUS. 61 



Diaptomus Castor is of common occurrence in ponds, 

 lakes, and ditches ; its colour and perhaps its varia- 

 tions of structure probably depend much upon the 

 character and quantity of its food as well as upon dis- 

 tinctions of race. The largest specimens are usually 

 found in ditches and rather foul, weedy water ; those 

 of mountain tarns and lakes are, as a general rule, 

 considerably smaller, not exceeding xy th or -^th of an 

 inch in length. The most highly-coloured specimens 

 that I have seen were taken in Grisedale Tarn, under 

 Helvellyn, and are referred to above as being of a 

 brilliant vermilion-red. The form described by Sir 

 John Lubbock under the specific name Westivoodii does 

 not appear to me sufficiently well defined or permanent 

 in its characters to warrant its separation as a distinct 

 species ; it is indeed, so far as my observation goes, 

 much commoner than the typical JD. Castor. The 

 characters upon which Sir J. Lubbock lays most stress 

 are, 1st, those of the fifth pair of feet, which do not 

 present any strongly-marked difference except as to 

 the inner branch of that of the female (fig. 11), which 

 is better developed than in the Castor form; 2ndly, 

 in the angulation of the last thoracic segment, which 

 after all appears to me to be nearly as marked in one form 

 as in the other ; 3rdly, in the length and spinous 

 armature of the antennas, both of which are liable 

 to a good deal of variation; but the spine on the 

 antepenultimate joint of the antenna of D. Westivoodii 

 I have found in some cases to coexist with the other 

 distinctive characters of D. Castor. For these reasons 

 I think it best to unite both forms under the old 



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