SENSE ORGANS. 



635 



with associated ganglia arises from the tritocerebrum and sup- 

 plies the labrum and alimentary canal. It is partly median in 

 position. 



eyes of insects are of two kinds, 



gh I 



pz 



FIG. 392. Section through the eye of a Coleopterous 

 larva (Dytiscus) (after Grenacher, from Hatschek's 

 Text-book), c chitinous cuticle ; / cornea! lens ; 

 h hypodermis ; pz pigment-cells ; gk vitreous 

 body ; r retina ; b basal membrane. 



Sense Organs, (i) The 

 compound and simple. 

 The compound eyes are 

 sometimes regarded as ho- 

 mologous with a pair of 

 appendages borne on the 

 most anterior segment of 

 the head. They consist of 

 a number of ommatidia 

 (v. p. 329), sometimes many 

 thousand, and may occupy 

 a very large area of the 

 surface of the head. They 

 may as in the case of the blind termites and in many cave-insects 

 be altogether suppressed. In the Diptera and Hymenoptera and 

 others, in which the eyes reach a high degree of perfection, there 

 is a distinct portion of the brain, the optic tract, from which 

 the optic nerves originate. The simple eyes are usually two or 

 three in number, but may be as many as twenty, grouped on 

 each side of the head. They are entirely wanting in the Der- 

 maptera and in certain Hemiptera and some other insects, and 

 to some extent their presence is correlated with a well-developed 



power of flight. 



(ii) The olfactory 

 organs of insects ap- 

 pear to reside in the 

 antennae, and their 

 terminal str u c t u r e s 

 take the form of 

 minute hairs partially 

 sunk in microscopic 

 pits. These exist in 

 extraordinary numbers ; for instance each antenna of the male 

 cockchafer is said to be provided with some forty thousand of 

 these organs. 



(iii) The auditory organs. In comparatively few insects are 

 there well developed, specialized auditory organs. Certain 



FIG. 393. Head, thorax and abdomen of an Acridium 

 seen from the side. St stigmata ; T tympanic organ. 



