I AQUATIC BEETLES 45 



ment, which has been repeated many times in text- 

 books and popular treatises both in old times and 

 very recently. 



Meinert ^ a few years ago was led to investigate all 

 these larvae afresh. In Myrmeleon and Hemerobius 

 he found that the mandibles are not truly perforated, 

 but only grooved along the inner edge. The groove 

 is converted into a tube by the maxilla. Sections 

 prove that there is a narrow cleft in the usual position 

 of the mouth. In the Dytiscus-larva too a vertical 

 section from back to front reveals the existence of a 

 mouth-passage, and Meinert showed that pressure on 

 the head not only causes the contents of the alimen- 

 tary canal to flow out drop by drop from the orifices 

 in the mandibles, but also causes an exudation from 

 the true mouth. He passed a fine hair into the slit 

 at the tip of the mandible, and found that it came 

 out at the base, where Westwood had long ago figured 

 a second orifice. In the Dytiscus larva the mandible 

 is traversed by a complete tube, into which the maxilla 

 does not enter at all. 



Meinert's description and explanation were fiercely 

 disputed by Schiodte, but confirmed by Dewitz and 

 Redtenbacher. Burgess- adds some curious details 

 of structure. He confirms the existence of a narrow 

 passage between the upper and under lips, but finds 

 that this is ordinarily closed by a " mouth-lock " — a 

 grooved joint such as is used for dust-proof doors. 



^ " Om Mundens Bygning hos Larverne af Myrmeleontiderne, 

 Hemerobierne og Dytiscerne." Vui. Medd. Nat. Foreti., 1879, 

 p. 69. 



2 Proc. Bostofi Soc. Nat. Hist., Vol. XXI. p. 223 (1881). 



