154 MORPHOLOGY OF INVERTEBRATE TYPES 



tympanal organs of the first abdominal somite, while the two 

 facetted and three simple eyes are situated on the head. The 

 sense of touch is highly developed in the antennae, while nerve 

 endings for the perception of taste are found chiefly on the hy- 

 popharynx. We shall consider only the eyes and the tympanal 

 or auditory organs. The facetted or compound eyes are composed 

 of a great number of ommatidia. An ommatidium is composed 

 of a chitinous cornea, a crystal cone, pigment cells which isolate 

 one ommatidium from another and prevent reflections of rays 

 forming too great an angle with the axis, and finally the retinula 

 with a central rod or rhabdome. The image formed by each 

 ommatidium correspond to a small portion of the horizon only, 

 and the actual image is a composite of the images formed by all 

 facets. The retina is direct. 



The simple eyes or ocelli are composed of a lens formed by the 

 cornea, a vitreous body which is rather poorly developed, and a 

 direct retina. The opinion has been recently advanced that the 

 function of the ocelli is to produce stereoscopic vision in con- 

 junction with the compound eyes, but this opinion is far from 

 being generally accepted. 



The tympanal or auditory organs belong to the type of so-called 

 chordotonal organs of which there are many varieties in insects. 

 They are situated on the first abdominal tergite and are rather 

 large. In typical chordotonal organs the swinging or vibrating 

 element is furnished by a tendon, while in a tympanal organ it 

 is a membrane-tympanum, stretched within an almost circu- 

 lar chitinous wall or rim. The nervous apparatus of the organ 

 is very complicated and apparently allows of a fine perception 

 of various sounds. 



Reproductive system. The sexes are separate and easily 

 recognizable owing to the presence of the ovipositor in the fe- 

 male. The female has two ovaries composed of a number of 

 tubes which sit in a row on the corresponding oviduct. The 

 two oviducts unite in a short vagina which opens between the 

 ovipositor on the subgenital plate (eighth sternite). A sperm 



