OF THE WATKll-BEETLE PELOBIUS TARDUS. 87 



or less globiilav structures with a deep cup-like pit in them, right 

 up to the time of the escape of the larva from the egg, and cjin be 

 seen, especially after the staining I'eferred to, after the larva has 

 hatched, disappearing as it expands. 



I had somewhat similar experiences as to the staining powei'S 

 of methylene blue in the case of the enibiyos oi D ytiscus lajijjonictis, 

 and I used to run a kind of hospital of methylene-blue tumblers 

 for the eggs. But in the case of that species I found another use 

 for methylene blue. When reai'ing the larvpe 1 found that I 

 could not keep them alive in tumblers, one in each, unless the 

 water was so charged with the stain that it looked like ink, in 

 which case the larvoe grew up quite successfully. This solution 

 was changed every two or three days, but in no case was I suc- 

 cessful with the larvfe in tumblers of plain water, even when I 

 changed the water daily. Possibly the explanation in this case 

 is that the methylene blue oxidized the faecal products and 

 remnants of food of the larvae and made them innocuous. 



4 (d) The Larva. 



The larva, on hatching, is colourless and possesses a long 

 medio-dorsal spine on the last abdominal segment and two 

 lateral cerci. Within the egg these structures are doubled 

 beneath the embryo, the last abdominal segments occupying the 

 end of the egg. On escaping from the egg, the larva at once 

 straightens out, and these appendages are at first about equal to 

 the body-length, though before the end of the first stage the body 

 has increased in length while the appendages remain the same. 



Within an hour the chitin hardens and the pigment has 

 appeared and darkened. The main colour of the larva is 

 brownish green, but there is a pattern of pale yellow, and this 

 varies in different individuals, some of which might be described 

 as almost black with a few light markings, while others are pale 

 yellowish with a few dark markings. The colouring apparently 

 does not vary much during the life of the individual — that is, 

 one which is dark in the first stage will be dark in its final stage. 

 An infinite number of diagrams could be given to show tlie 

 range from dark to light, but I have selected five specimens, two 

 at the end of their first stage (Nos. 1 and 2) and three in their 

 third stage (Nos. 3, 4, and 5) to show the range {vide PI. T. 

 figs. 1 to 5). 



A study of these diagrams will make it clear that, although 

 variations occur, the disappearance of the dark pigment in the 

 lighter individuals is not a phenomenon which may happen at 

 any spot on the surface of a segment, but occurs in more or less 

 definite areas; and this is seen even better by comparing a 

 number of individuals of about the same degree of darkness or 

 lightness when a rather surprising uniformity of pattern is 

 recognizable. 



There is, I think, some very interesting work to be done upon 



