240 



INVERTEBRATE ZOOLOGY 



consists of a central plate from which two or three pairs of arms 

 pass to the exoskeleton of the head. 



Sensory Organs. — Both simple (Fig. 117) and compound eyes 

 are found in the insects. Compound eyes occur on the sides of 

 the head in most adult insects except some generalized and some 

 parasitic forms. Practically all insects which have a complete 

 metamorphosis (the Holometabola) have simple eyes in the 

 larval stage. 



Aside from the eyes, other sensory organs in insects show 

 remarkable lack of uniformity in location upon the body and in 

 localization and organization. Even the antennae, which are 

 popularly thought of as tactile, have in some species of insects 

 no less than seven different types of microscopic sensory organs. 

 Most of these are probably tactile, auditory, and olfactory, 



though frequently it has been 

 necessary to assume functions 

 upon the basis of structure and 

 position and by interpretation of 

 the reactions of the insect rather 

 than to determine them by actual 

 demonstration. Hairs of gradu- 

 ated lengths upon the feathery 

 antennae of mosquitoes and moths 

 vibrate in response to sound 

 waves through a range which in 

 the mosquito coincides with the 

 pitch of the mosquito hum. 

 Another type of auditory organ, 

 of widely different structure and 

 location, is the tympanal organ, 

 which consists of a drumlike membrane for the reception of 

 sound waves. In the common grasshoppers, this tympanal 

 organ is located on each side of the first abdominal somite, while 

 in katydids a similarly constructed organ is found on the second 

 joint of each front leg. 



End organs of taste and smell are usually located on the maxil- 

 lary palpi, the epipharynx, the hypopharynx, and the labial 

 palpi, but they are not restricted to any given organ or appendage. 

 Tactile organs and sensory structures of undetermined func- 

 tions occur as modifications of the body covering over most of 

 the surface of an insect. 



Fig. 117. — Median ocellus of 

 beetle, Acilius. (After Patten) 



