18 



EYES. NERVOUS SYSTEM. 



29. In some di'pterous insects, in place of the second pair 

 of wings we find two pedunculated globular bodies, named hal- 

 teres, or poisers. 



30. The eyes of insects are always on a level with the head, 

 and are never borne on a movable peduncle, as in certain crus- 

 taceans ; sometimes their structure is the same as in ara'chnidans, 

 and they are called simple eyes, or ocelli ; but in all insects 

 there exist, either conjointly with them or separately, compound 

 eyes, or eyes with facets. 



" The compound eyes of insects are two in number, situated on the lateral 

 aspects of the head, the form of each being more or less hemispherical. 

 When examined with a microscope, their surface is seen to be divided into 

 a multitude of hexagonal facets, between which minute hairs are generally 

 conspicuous. The number of facets, or corneae, for such in fact they are, 

 varies in different genera : thus, in the ant (For'mica) there are 50 ; in the 

 common house-fly (Musca domestica) 4,000 ; in some dragon-flies (Libellula) 

 upwards of 12,000. In butterflies (Papilio) 17,355 have been counted, and 

 some Coleopteroe possess the astonishing number of 25,088 distinct cor- 

 nese." T. Rymer Jones. 



31. Of the organs of smell and of 

 hearing in these animals we know 

 nothing. The nervous system is 

 composed of a chain of double 

 ganglia, arranged as has already 

 been described (fig. 2, page 11). 



32. The mouth is placed in the 

 anterior and inferior part of the 

 head ; but its form varies con- 

 siderably, accordingly as the ani- 

 mal is destined to feed on solid or 

 liquid substances. 



33. In the Tritores, or triturating 



insects, the mouth is composed, 1st, Fig. 4. MOUTH OF AN INSECT. 



of an upper lip ; 2d, of a pair of 



mandibles ; 3d, of a pair of jaws ; and 4th, of a lower lip. 



. m 



ma- 



Explanation of Fig. 4. Apparatus of mastication of a coleo'pterous in- 

 sec t ;_Za, the labrum ; m, the mandibles ; ma, the maxillse or jaws ; 

 p, maxillary palpi ; Zi, the labium, or lower lip; pi, the palpi of the la- 

 bium. 



29. What are halteres ? 



30. How are the eyes of insects situated ? How many kinds of eyes have 

 insects ? What are compound eyes ? 



31. Where are the organs of smell and of hearing situated ? How is the 

 nervous system of insects arranged ? 



32. What is the character of the mouth ? Where is it situated ? 



33. Of wnat parts does the mouth, in triturating insects, consist ? 



