THE IXTERXAL STRUCTURE OF AXTS. 59 



nates in a so-called pre-stomachal ganglion, which innervates the sur- 

 rounding wall of the crop and gizzard. The paired supraintestinal 

 sympathetic has an anterior and a posterior portion. The former con- 

 sists of the cesophageal ganglia, which lie on each side of the hypo- 

 cerebral ganglion. They are united with this by commissures and with 

 the tritocerebrum by connectives, and innervate the sides of the oesoph- 

 agus and crop. The posterior portion of the paired system is very 

 imperfectly known. Janet maintains that the fourth pair of nerves 

 from the terminal ganglion of the ventral cord turns forward and 

 innervates the posterior portion of the digestive tract in somewhat the 

 same manner as the anterior portion is innervated by the brain through 

 the frontal and cesophageal ganglia. He, therefore, calls these the 

 proctodseal recurrent nerves. The subintestinal sympathetic system of 

 Mvrnrica comprises a series of minute, unpaired, metameric ganglia 

 connected with several of the ganglia of the ventral cord. This system, 

 too, both in ants and in other insects, is imperfectly known. 



The Sense-Organs. The sense-organs of ants, like those of insects 

 in general, are modifications of the integument and the terminations of 

 senson- nerves. Hence there can be no sense-organs in the interior of 

 the body unless they have been carried in secondarily on in foldings of 

 the integument. As there are no openings anywhere in the chitinous 

 investment of the insect's body, except those at the anterior and pos- 

 terior ends of the midgut, the nerve terminations are never freely 

 exposed on the surface, but always covered with at least a very delicate 

 layer of chitin. The number and diversity of sense-organs in insects 

 is very great, but nevertheless, attempts have been made to trace them 

 all back to a common primitive type. One of the most recent of these 

 attempts is that of Berlese ( 1907') who finds that nearly all these organs 

 admit of hypothetical derivation from a " protaesthesis," a sensilla, or 

 sense-bud, consisting of one or a few chitin-secreting hypodermal cells, 

 a gland cell and a nerve cell. It is possible to show that this type of 

 structure keeps recurring in the various sense-organs of even sugh 

 highly-specialized insects as the ants. 



Tactile (Trichodeal) Sensillae. As stated in a previous chapter, 

 ants are usually covered with hairs, which are coarse and long on the 

 body and shorter and denser on the legs and especially on the antennae. 

 As all of these hairs are movably articulated to the general chitinous 

 integument and are provided with fine nerve terminations, they are 

 universally regarded as tactile sensilla, although they also aid in the 

 removal of the larval or pupal skin during ecdysis, for they are at 

 first bent at their bases and applied to the chitinous layer to which 

 they belong, but later, in becoming erect, loosen and push the overlying 



