WERNER MARCHAND 153 



ing food. These tentacles form the armature of the stomal disk, 

 consisting of a process arranged like a turnstile mounted on a pitted 

 chitinous plate at the base of the great hook or mandible. In the 

 very young larva the stomal disk appears as a chaplet of delicate 

 chitinous rods. When a larva is treated with strong caustic potash, 

 the stomal disk appears to be the only structure which resists its ac- 

 tion, the other chitinous structures, even the heavy pharyngeal 

 apophyses, are bleached. In common with the other chitinous 

 portions of the head capsule the stomal disk is shed at each of the 

 three ecdyses. 

 An account of the general development of the larva follows: 

 The young larva shows in its form and behavior its adaptability 

 to an aquatic life. This is well illustrated when a larva is placed in 

 an aquarium containing mosquito wrigglers. The Tahanus has no 

 difficulty in keeping afloat with them and foraging at will upon the 

 active culicid larvae. Tahanus larvae have been observed capturing 

 wrigglers, holding them by their jaws under the water, and actually 

 killing the cuHcid through drowning. In one instance a Tahanus 

 larva held its victim, which was fully five times its size, suspended 

 beneath it in such manner that the culicid was unable to project its 

 syphon for breathing purposes, while that of the Tahanus was func- 

 tional. The Tahanus, obtaining a secure perch by dragging itself 

 and the prey above the water, devoured the mosquito wriggler in a 

 few minutes. In another instance the weight of the culicid pulled 

 its captor under the water to the sandy bottom a distance of nearly 

 30 cm. Here the Tahanus showed its superior vitality by remaining 

 attached for nearly two minutes until apparently assured of the 

 immobility of its prey, then, releasing its hold, the Tahanus larva 

 struggled to the surface where it rested with syphon extended. The 

 mosquito larva meanwhile moved feebly several times, and suc- 

 cumbed within a few minutes. 



This adaptability is lost, however, in the developed larva which 

 becomes more slothful in movement and grub-like in superficial ap- 

 pearance. Both extremities, the head and the syphon, become ob- 

 tuse in form, and the ventral protuberances functioning as prolegs 

 become more truncated. Growth after the second molt becomes 

 noticeably less in length and more in thickness. The greatest growth 



