EXTERNAL ANATOMY OF INSECTS. 473 



form pieces a . In Melophagus, the sheep-louse, the union 

 of the valves of the sheath is so short, that they appear 

 like a tube ; but if cut off they will separate, and show 

 the siphuncle, as fine as a hair, between them. This or- 

 gan is of a type so dissimilar, as was before observed, to 

 that of the Diptera in general, and approaches so near to 

 that of the dog- tick [Ixodes), that they may be deemed ra- 

 ther apterous insects with two wings, than to belong to that 

 Order ; and the circumstance that some of the family are 

 apterous confirms this idea. In fact they are a transition 

 family that connects the two Orders, but are nearest to 

 the Aptera. In Nycteribia the oral organs differ from 

 those of the other Pupipara in having palpi. This also 

 is the case with those of the genus Ixodes, the palpi of 

 which are placed upon the same base with the instru- 

 ment of suction, than which they are longer : they ap- 

 pear to consist of two joints, the last very long and flat. 

 The instrument of suction itself is formed by three hard 

 rigid laminae ; two shorter parallel ones above, that co- 

 ver the third, which is longer and broader, and armed 

 on each side with several teeth like a saw, having their 

 points towards the base b . Many of the other Acari L. 

 have mandibles, and several have not : but their oral or- 

 gans have not yet been sufficiently examined ; and from 

 the extreme minuteness of most jf them, this is no easy 

 task; nor to ascertain in what points they differ or 

 agree. 



If you consider the general plan of the organs of man- 

 ducation in the vertebrate animals, how few are the va- 



a N. Diet. d'Hist. Nat. xxviii. 266. 



b Ibid, xvi- 432. De Geer vii. t.vi.f. 4. Not quite accurate. 



