332 MR FRANK BALFOUR-BROWNE ON 



We may, perhaps, also account for the "mast" on the cocoon of Hydrophilus and 

 the ribbon on that of Hydrohius as connected with this peculiar habit of drinking air, as 

 so far no satisfactory explanation has been given of these structures. They have been 

 described as the result of excessive zeal on the part of the female — a sort of using up 

 of silk, and the mast of the Hydrophilus cocoon has also been described as an air-funnel 

 to aerate the eggs. The mast always projects above the water, while the ribbon is either 

 spread out upon the surface or upon some object above the surface. 



The larvae of one batch of eggs do not all necessarily hatch at the same time, some 

 of the eggs — probably the upper ones — hatching sooner than the others. Now the 

 earliest hatched larvae presumably take their drink of air, and as they remain some time 

 in the cocoon, they no doubt tend to use up the limited supply within the cocoon. 

 When they escape they open the cocoon below or on the water-line, and there is there- 

 fore a risk of the remaining air escaping. I think the mast and the ribbon are 

 provided against all these possibilities, and to make certain of other larvae getting to 

 the surface to obtain their supply of air. 



The head of the larva has two eyes made up of about six more or less oval patches 

 of pigment. The antennas are short, three-segmented organs, the apical segment being 

 much finer than the others. The other two segments are, in the youngest stage, about 

 equal in length and twice that of the apical one, but in the growth of the antennae the 

 basal segment grows more rapidly than the middle one, so that, in the final stage, the 

 basal segment is about twice the length of the middle one. On the middle segment and 

 just external to the insertion of the apical one is a small tubercle. The apex of the 

 terminal segment bears two long hairs. 



The mandibles are large, and end in a sharp incurved point and bear three sharp 

 teeth on their inner margin, the basal one being small. The two mandibles are quite 

 similar to one another. The maxillae are long and are presumably to be described as 

 consisting each of a cardo and stipes bearing a minute inner lobe — the undifferentiated 

 galea and lacinia — and a large three-segmented palpus. The middle segment of the 

 latter bears at its apex, just external to the insertion of the apical segment, a long 

 fine hair. 



The labium consists of a broad submentum and mentum, the latter bearing a pair of 

 two-segmented palpi, and between them a small process presumably corresponding to a 

 ligula. The submentum on its upper surface bears series of closely set minute outwardly 

 projecting points. 



The labrum is peculiar. Its free edge is strongly serrated in the middle, but the 

 organ is not bilaterally symmetrical. The central part of the anterior margin bears 

 about five teeth. Some slight variations occur in the arrangement of the teeth, and 

 they also change slightly at the moults, but the general arrangement is that there are 

 five teeth of which the four on the right are more or less similar and evenly spaced, 

 while the one on the left is widely separated from the rest of the series. This want of 

 bilateral symmetry of the labium is not unique in Hydrohius, as it occurs also in 



