334 MR FRANK BALFOUR-BROWNE ON 



a three-lobed expansion, below the middle lobe of which the anus opens, and above 

 which is a depression bounded anteriorly by a more or less perpendicular wall of tissue 

 in the form of a slightly lobed ridge. Right and left of this ridge and just posterior 

 to it is a small lobe bearing a flattened chitinised projection on which are three long 

 hairs. The trilobed expansion is fringed with several long hairs and bears on its upper 

 surface, just at the junction of the middle with the lateral lobes, two dark-coloured 

 papillae each with two long hairs at its apex and one long hair projecting from a small 

 papilla on its outer side. # 



On the posterior face of the anterior ridge and at either side of the median line is a 

 narrow aperture — the spiracle — leading into a small chitinous vestibule into which opens 

 the lateral tracheal trunk. The trunk is constricted where it opens into the vestibule, 

 and there appears to be some apparatus of small chitinous pieces controlling this opening. 



The lateral lobes and the trilobed part of the segment are capable of spreading 

 outward and of closing in, thus enlarging and diminishing the dorsal depression accord- 

 ing as the larva is obtaining air at the surface and holding on to the surface film or 

 moving under water. 



On the dorsum of this second part of the last abdominal segment is a large semi- 

 circular chitinised tergite, developed perhaps for the protection of the functional spiracles 

 which open just below its posterior margin. 



The surface of the skin of the larva is covered with scattered hairs arising out of 

 small papillae amongst which are numerous small pores leading into short pits each of 

 which gives off into the skin numerous minute hair-like processes. Amongst these pits 

 are still more numerous minute pointed pits which look very like minute inverted spines. 



The larvae are metapneustic, the only functional spiracles being those opening into 

 the dorsal depression of the eighth abdominal segment. The expansion of the lobes of 

 this segment enable the larva to hold on to the surface film, the rest of its body being 

 submerged. By moving the body rapidly from side to side when thus attached to the 

 surface, the larva moves fairly rapidly ; otherwise its only method of progress is by 

 creeping on the water plants, as it has no power of free swimming like the young larva of 

 Hydrophilus. 



The position of the larva in relation to the surface film has nothing to do with the 

 state of buoyancy, since an individual with no air in its gut can and does travel suspended 

 to the surface film as easily as one filled with air ; but whereas the former sinks if, from 

 any cause, it loses its hold, the latter floats and can regain its attachment to the film. 



The method of feeding of the larva is peculiar. Its habit is to lie half out of the 



* Between these two papillae is a small median pit from which bubbles of air seem to come off if, for instance, the 

 larva is placed in alcohol. I have seen the same phenomenon in the case of the larva of Hydrocharis caraboides, L., and 

 it would seem to indicate that there is a median spiracle or pair of spiracles opening at this point, although I cannot 

 see any trachea; leading there. But by putting the larva in water under the microscope and flooding with alcohol, air- 

 bubblesseem to come oil" from numerous rdaces. For instance, in one or two cases I distinctly saw bubbles escaping 

 from the tracheal papillae of the first abdominal segment, and in many cases bubbles come from the sides of the 

 other segments, but apparently not from the tracheal papillae. Where these latter bubbles come from I cannot 

 determine, and at present I can merely mention these observations, leaving the matter for a future occasion. 



