238 



Arthropoda, 



and all tlie abdominal ganglia may unite to form a single mass, 

 which, in extreme cases, includes also the first thoracic ganglion. 



Sense organs. Olfactory organs occur as short, delicate, 

 thin-walled hairs, which receive filiform processes from sense-cells 

 lying beneath them (Fig. 18) ; they occur on the antenna, often situated 

 in pits. Auditory organs probably occur in the majority of In- 

 sects, since there is direct proof that many can perceive sounds ; and 

 indirectly, it is probable that, since many can produce noises, they can 

 also perceive them ; these organs are, however, at present only known 

 with certainty for quite a few forms. In the Grrasshoppers, there is 

 on the side of the first abdominal segment a thin membrane (the 

 "tympanum," a specially developed portion of the skin), stretched at 

 the bottom of a depression. Beneath the membrane there are peculiar 

 cells, each inclosing a delicate pin-like body, and connected with a 

 nerve fibre. It is believed that the membrane is caused to vibrate by 

 sound waves, and that this reacts on the cells described ; the sound 

 is intensified by a tracheal bladder, which lies close to the tympanum, 

 and serves as a resonator. Auditory organs of a somewhat different 

 structure occur in the Locusts on the tibiae of the first pair of legs. 

 In other cases there are cells like those described above, but without 

 tympanum or resonator, and it is supposed that these may also be 

 regarded as simple auditory apparatus. For eyes, the account on 

 p. 231, and the description of the structure of Arthropod eyes given 

 in the General Part, may be consulted. 



Alimentary canal. In Insects with sucking mouth-parts, 

 strong muscles run from the buccal cavity to the inner side of the 

 head : the cavity of the mouth is enlarged by their contraction, and 

 thus the fluid into which the proboscis is dipped is drawn in. One or 



more pairs of salivary 

 glands open into the mouth. 

 The rest of the alimentary 

 canal, which may be 

 straight or looped, is made 

 up of the oesophagus, the 

 mesenteron, and the proc- 

 todseum. The oesophagus 

 is usually narrow in front, 

 but swells out behind into 

 a crop, which is either 

 a simple dilation or a special pouch-like appendage connected with 

 the rest of the oesophagus by a narrow duct; this is the case in 

 many suctorial Insects. The crop serves as a reservoir for the food. 

 Occasionally the terminal part of the oesophagus is particularly 

 muscular, provided with hard parts on the inner surface, and 

 serves as a gizzard. The mesenteron is the essential digestive 

 portion, though the secretion of the salivary glands assists in this 



Fig. 200. Diagrammatic longitudinal section of 

 the head of an insect with sucking mouth-parts. 

 su sucking tube, m buccal cavity, mu muscles which 

 widen the latter, ce oesophagus. — Orig. 



