ITS OUTER SKELETON. 47 



roaches has satisfied us that the palps are constantly used when 

 the Insect is active, whether feeding or not, to explore the 

 surface upon which it moves. "We have seen no ground for 

 attributing to the palps special powers of perceiving odours or 

 flavours, nor have we observed that they aid directly in filling 

 the mouth with food. 



It is worthy of note that Leydig has described and figured 



ti fc' <j t j 



in the larva of Hydroporus (?), and Hauser in Dytiscus, 

 Curabits, &c., a peculiar organ, apparently sensor}-, which is 

 lodged in the maxillary and labial palps. It consists of whitish 

 spots, sometimes visible to the naked eye, characterised by 

 unusual thinness of the chitinous cuticle and by the aggregation 

 beneath it of a crowd of extremely minute sensory rods. Of 

 this organ no satisfactory explanation has yet been given.* 



Comparison of Mouth-parts in different Insects. 



The jaws of the Cockroach form an excellent standard of 

 comparison for those of other Insects, and we shall attempt to 

 illustrate the chief variations by referring them to this type.f 

 Mouth-parts are so extensively used in the classification of 

 Insects that every entomologist ought to have a rational as well 

 as a technical knowledge of their comparative structure. ]STo 

 part of Insect anatomy affords more striking examples of 

 adaptive modification. In form, size, and mode of application 

 the jaws vary extremely. It would be hard to find feeding- 

 organs more unlike, at first sight, than the stylets of a Gnat and 

 the proboscis of a Moth, yet the study of a few well-selected 

 types will satisfy the observer that both are capable of deriva- 

 tion from a common plan. Nor is this common plan at all 

 vague. It is accurately pictured in the jaws of the Cockroach 

 and other Orthoptera. These correspond so entirely with the 

 primitive arrangement, inferred by a process of abstraction from 



* Ley dig, Taf. z. vergl. Anat., pi. x., fig. 3. Hauser, Zeits. f. wiss. Zool., Bd. 

 XXXIV., p. 386. Jobert has figured the sensory organs of the maxillary palps of 

 the Mole-cricket (Ann. Sci. Nat., 1872), and Forel similar organs in Ants (Bull. Soc. 

 Vaudoise, 1885). 



"t* The reader who desires to follow this subject further is recommended to study 

 chap. vi. of Graber's Insekten, which we have found very useful. 



