TYPE TBAOHEATA. 495 



maxillae, and the labium ; (3) of three pairs of ganglia in the 

 corresponding thoracic segments ; and (4) of a chain of ab- 

 dominal ganglia, a pair occurring in each segment except 

 usually the last two or three, in which a compound ganglion 

 occurs. Frequently, however, this typical condition is modi- 

 fied by a greater or less concentration of the various ganglia, 

 the thoracic ganglia fusing to a single mass, as may also, more 

 or less perfectly, the ganglia of the abdominal chain (Fig. 228, 

 £), and the latter may even unite with the thoracic ganglia to 

 form a single mass situated in the thorax, as in certain two- 

 winged flies (Fig. 228, C). A visceral system is usually pres- 

 ent arising from the supraoesophageal (cerebral) mass and 

 being distributed to the walls of the digestive tract. 



The antennae of insects seem to act as sense-organs, and 

 serve apparently to control the flight, since when removed the 

 insect is not able to fly with its accustomed ability. So too 

 it seems probable that in the Ants and Roaches these appen- 

 dages are the seat of the olfactory sense, and in the Mosquito 

 it seems that certain hairs upon them may be auditory in 

 function. Compound eyes, frequently consisting of several 

 thousand ommatidia, are usually present, as well as a small 

 number of simple eyes (ocelli) situated upon the dorsal sur- 

 face of the head. Special organs, which have usually been 

 considered auditory, also occur in many, forms, varying con- 

 siderably in complexity. In its simple form such an organ 

 consists of a single nerve-fibre which dilates into a ganglion- 

 cell, prolonged into a terminal hair which is enclosed within 

 a sheath fastened at one end to the wall of the body. This 

 whole apparatus is termed a chordotonal organ, and there is 

 usually attached to the sheath Just where the hair arises from 

 the ganglion a ligament, which is also inserted into the body- 

 wall. In the majority of cases a number of ganglion-cells and 

 hairs are associated to form a chordotonal organ (Fig. 229), 

 the various hairs sometimes being grouped within a single 

 sheath, — sometimes, however, spreading out in a fanlike man- 

 ner, each possessing its own sheath. These organs occur in vari 

 ous parts of the body, on the antennae or on the limbs. In 

 the grasshoppers (Acridiidae) the first abdominal segment 

 bears on either side a thin tense membrane, a thinned portion 



