ANIMAL BEHAVIOR, WITH EXAMPLES 169 



The nervous system being the mechanism by which the actions 

 or behavior of animals are directed, it is the special sense organs 

 that are the windows through which the outer world is per- 

 ceived. I give here, in diagrammatic form, these organs in 

 insects and their anatomic disposition as they are at present 

 interpreted : 



Paets of the Insect's Body 



Eyes Simple, with single lens (ocellus). 



Compound or facetted eye. 



Antennw. Olfactory (smell) organs with pore canal, having a sunken 



or free hair-like structure attached to a slender nerve 



fibre, and connected with many nucleated ganglion cells; 



or often presenting a tooth-like projection filled with serous 



Head \ fluid. Antennal auditory hairs also appear. 



Mouth. . . . Within, or immediately surrounding it, bearing organs of 

 taste: (a) proboscis or tongue. Taste (distinguished with 

 diflBculty from olfactory). The tip of proboscis; the 

 epipharynx in nearly all insects have taste buds; the 

 maxillae of wasps have taste cones. (6) Clypeus. At front 

 edge bear taste buds in Orthoptera. 



Thorax. . .At base of wings of many insects club-shaped rods appear, 

 supposed to be auditory organs, also perception of move- 

 ment of halterers in Diptera. 



Legs Tibia of grasshoppers, bearing chordotonal organs, also 



in ants, Perlidse, etc. 



Abdomen, .^asal joint, on each side, just behind spiracle. Organ of 

 hearing, connecting with the third thoracic ganglia. 

 Common in Orthoptera. 



Cerci (at the end of the abdomen) hearing organs 

 (Packard). 



In a former chapter, under Evolution and Instinct, the sub- 

 jects of reflex action, instinct, and reason have already been 

 discussed. Habit, as ordinarily supposed, is a voluntary action 

 repeated until it becomes reflex. It is essentially like instinct 

 in aU of its manifestations, but instinct is inherited habit. 

 The sensorium is offered a choice of responses. To choose 

 one and reject the others is the function of intellect or reason. 

 But granting these distinctions, instinct and reason are not 

 sharply defined. 



