THE MOUTH OF INSECTS 989 



small circles that beset almost their entire area ; their form, which 

 is very peculiar, can here be only made out by vertical sections ; but 

 in many of the smaller antenna?, such as those of the bee, the 

 cavities can be seen sidewise without any other trouble than that 

 of bleaching the specimen to render it more transparent. 1 



The next point in the organisation of insects to which the atten- 

 tion of the microscopist may be directed is the structure of the 

 mouth. Here, again, we find almost infinite varieties in the details 

 of conformation ; but these may be for the most part reduced to a 

 small number of types or plans, which are characteristic of the dif- 

 ferent orders of insects. It is among the Coleoptera, or beetles, that 

 we find the several parts of which the mouth is composed in their 

 most distinct form ; for, although some of these parts are much more 

 highly developed in other insects, other parts may be so much altered 

 or so little developed as to be scarcely recognisable. The Coleoptera 

 present the typical conformation of the mandibulate mouth, which is 

 adapted for the prehension and division of solid substances ; and this 

 consists of the following parts : 1, a pair of jaws, termed mandibles, 

 frequently furnished with powerful teeth, opening laterally on either 

 side of the mouth, and serving as the chief instruments of manduca- 

 tion ; 2, a second pair of jaws, termed maxillce, smaller and weaker 

 than the preceding, beneath which they are placed, and serving to 

 hold the food, and to convey it to the back of the mouth ; 3, an 

 upper lip, or labrum ; 4, a lower lip or labium ; 5, one or two pairs 

 of small jointed appendages, termed palpi, attached to the maxilla?, 

 and hence called maxillary palpi ; 6, a pair of labial palpi. The 

 labium 2 is often composed of several distinct parts, its basal portion 

 being distinguished as the mentum or chin, and its anterior portion 

 being sometimes considerably prolonged forwards, so as to form an 

 organ which is properly designated the ligula, but which is more 

 commonly known as the ' tongue,' though -not really entitled to that 

 designation, the real tongue being a soft and projecting organ which 

 forms the floor of the mouth, and which is only found as a distinct 

 part in a comparatively small number of insects, as the cricket. This 

 ligula is extremely developed in the fly kind, in which it forms the 

 chief part of what is commonly called the ' proboscis ' (fig. 739) ; 3 



1 See the memoir of Dr. Hicks, ' On a new Structure in the Antennae of Insects,' 

 in Trans. Linn. Soc. xxii. p. 147; and his 'Further Remarks' at p. 383 of the 

 same volume. See also the memoir of M. Lespes, ' Sur 1'Appareil auditif des Insectes,' 

 ioi.Aim.de9 Sci. Nat. ser. iv. Zool. torn. ix. p. 258 ; and that of M. Claparede, ' Sur le's 

 pretendus Organes auditifs des Coleopteres lamellicornes et autres Insectes,' in Ann. 

 des Sci. Nat. se"r. iv. Zool. torn. x. p. 236. Dr. Hicks lays great stress on the 'bleach- 

 ing process ' as essential to success in this investigation, and he gives the following 

 directions for performing it : Take of chlorate of potass a drachm, and of water a 

 drachm and a half ; mix these in a small wide bottle containing about an ounce ; wait 

 five minutes and then add about a drachm and a half of strong hydrochloric acid. 

 Chlorine is thus slowly developed, and the mixture will retain its bleaching power 

 for some time. For an account of Herr F. Ruland's observations see Joum. Hoy 

 Micr Soc. 1888, p. 723. 



2 The labium and the labial palps are, morphologically, a second pair of maxillae 

 which have undergone more or less fusion of the basal parts along the median line. 



"' The representation given in the figure is taken from one of the ordinary pre- 

 parations of the fly's proboscis, which is made by slitting it open, flattening it out, 

 and mounting it in balsam. For representations of the true relative positions of 

 the different parts of this wonderful organ, and for minute descriptions of them, the 



