COLEOPTERA. — PETALOCEKA. 193 



plates, which shut and open like the leaves of a book : in some spe- 

 cies these plates are flat, so as to be placed side by side {fig. 20. 18., 

 22. 8, 9.) ; but in others the outer plates are, or, at least, the basal 

 one is, hollowed, forming a kind of box, enclosing the intermediate 

 plate {fig, 20. 7. i . :. >.) ; the mandibles, which are very variable in their 

 structure, corresponding with the habits of the various groups, do not 

 exhibit an}- striking sexual variations ; but the head and thorax of 

 the males, in some of the groups, arc armed with strange horns or 

 protuberances of the most diversified appearance, and of which it is 

 very difficult to conceive the uses ; the antennte, also, in the same sex, 

 are sometimes more developed than those of the females ; these or- 

 gans are inserted in cavities at the lateral margins of the head ; the 

 palpi are short and filiform ; the mentum is large and horny {fig. 20. 

 5, 6., 22. 2. XX.), often covering the labium (or ligida), and sometimes 

 preceded by a transverse piece {fig- 20. 5, 6., 22. 2. x.), which has been 

 named by ^NlacLeay the stipes, and bj' Strauss la piece prebasilaire, 

 but which does not constitute a distinct articulation, being only the 

 front part of the jugulum {fig. 20. 5, 6. o.) ; although in some species, 

 which appear to be destitute of a labium, which is internal, and of 

 which the place is supplied externally by the mentum, the stipes is 

 more fully developed, so as to appear to supply the place of the mentum, 

 as in the Cockchafer {fig. 22. 2.) ; the body is generally more or less 

 oval and convex, the legs robust, the anterior, and sometimes all, the 

 tibiae, generally toothed on the outside. In many of these insects, espe- 

 cially those which feed upon leaves, the internal edge of the mandibles 

 is formed into a broad horny plate, with various transverse channels, 

 well formed for masticating. This structure has been described by 

 Knoch (^Xeiie Beitrage, vol. i.), Olivier (^Entomol. i. 6. 42.), and by 

 Kirby (Lhui. Trans, vol. 14.). (See my fig. 22. 5.) 



Some singular monstrosities, occurring in various species of this 

 family, have been recently described ; namely, Rhisotrogus (A?m. Soc. 



Olivier, in Journ. d'Hist. Nat. vols. i. and ii. 



Schrank. Uebcr die Kafergattung Melolontha (Physic. Arbeit, der Eintracht. fr. in 



Wien. ann. 2.) 

 Fieber. Die biihmischen und osterrcischs. Cetonien (Jahrb. d. Ges. bijlim, iMiiseums 



2 Jlihrg.) 

 Siickou: Naturg. der Insccten, 8vo. Heidelberg, 181S. 

 Oliiiei; Gt/lloihaU, Sc/ionherr, I{erbst, Jablonski/, ^-c. 



O 



