INSECT A. 149 



lific stomach." From this there proceeds an intestine (/*), 

 of variable length, which usually terminates in a chamber (g) 

 called the " cloaca " (Lat. cloaca, a sink), into which the ducts 

 of the reproductive organs open. The commencement of 

 the gullet is furnished with glandular appendages, which are 

 believed to discharge the functions of salivary glands. Imme- 

 diately behind the posterior aperture of the stomach are a 

 variable number of caecal convoluted tubes (e), which are 

 known as the " Malpighian vessels," after their discoverer Mal- 

 pighi, and which are generally looked upon as representing the 

 liver. Close to the cloaca may be other tubes, which are be- 

 lieved, from their position, to exercise the functions of kid- 

 neys (A). 



The circulation in insects is mainly carried on by a long, 

 contractile tube, placed along the back, and termed the " dor- 

 sal vessel." The blood, collected from the various tissues and 

 organs of the body, enters the dorsal vessel from behind, and 

 is driven forward to the anterior extremity of the body. Res- 

 piration is effected by means of air-tubes or tracheae, which 

 commence at the surface by so many apertures or spiracles, 

 and branch repeatedly as they proceed inward through the 

 tissues. They have essentially the same structure as in the 

 Arachnida, consisting of membranous tubes strengthened by 

 means of a spirally-coiled filament of chitine. 



The nervous system in insects, though sometimes some- 

 what modified, has essentially the regular annulose form of a 

 ventral chain of ganglia, traversed in front by the gullet. The 

 organs of sense are the eyes and antennae. The eyes are 

 usually " compound," and are composed of numerous six-sided 

 lenses, united together, and each supplied by a separate ner- 

 vous filament. As many as eight thousand of these lenses 

 have been counted in one of the eyes of the common cock- 

 chafer, and this number is sometimes greatly exceeded. Be- 

 sides these compound eyes there are sometimes "simple" 

 eyes, identical in structure with the single lenses of the com- 

 pound eyes ; and in rare cases these are the only organs of 

 vision. The feelers or antennae, with which all insects are 

 furnished, are jointed filaments attached close to the eyes, and 

 assuming very different shapes in different insects. They ap- 

 pear to be certainly organs of touch, but they very probably 

 minister to other senses as well, and there is some reason to 

 suppose that they are connected with the sense of hearing in 

 particular. 



The sexes in insects are distinct, and most of them are 



