300 KATURAL IHSTOnY OF ARTHROPODS. 



tions ill carnivorous ln'ctles :m(l innif coiivcilutiniis in licrliivorous ones, may be 

 designated as the veiitriele. The posterior end of the ventricle is marked hy the 

 insertion of four or six long tubes, the mal|iiuliian vessels, which are urine-excreting 

 organs. IJcyoiid the insertion of the maliiighiau vessels, and ending at the amis, 

 is the intestine, ]iroperly speaking, often showing a differentiation into small intestine, 

 Cfficuni, and recluni. The maliiighian tul)es are usually a iiumlier of times as long as 

 the insect itself, and are coiled aVjout in the abdoineii <if the beetk' in a way that makes 

 them very difficult to disentangle without breaking. (tL-ukIs ojieii into various portions 

 of the digestive tract, among them rectal glands into the rectum. C'arabidie, Dytis- 

 cidse, and some other beetles have glands oiieiiing on each side of the anus that secrete 

 acid or strongly odorous iluiiis which are used by these insects tn defend tliemselves. 

 Ill the bombardier-beetle (linir/iiniiK) the secretion of these anal glands is partly 

 gaseous, or becmnes aeriform iiiiinediately after its discharge. 



The nervous system consists of a supra-iesoiihageal ganglion, from wliicli originate 

 the antennal and optic nerves; an iiifra-(esoiihageal ganglion which sends nerves to 

 the mandibles, maxilhv, and labium ; ami a series of ganglia, connected by double 

 commissures, just beneath the digeslixe tract. During the growth of the larvre these 

 double ventral ganglia, typically distinct and one in each segment, consolidate in 

 various way.s, so that, in the iinagos, Eduard Hiaiidt has characterized four types of the 

 nervous systemi, as follows: first, a system with .1 supra-iesophageal ganglion and a 

 large central nerve-mass in the thi:)rax, of uliicli mass tlie forward end is the infra- 

 tt'sojihageal ganglion; secoml, a system with two cephalic ganglia, and a central 

 nerve-mass in the thorax; third, a system with two cephalic, two thoracic, and none 

 to eight abiloiuinal ganglia; fourth, a system with two cejihalic, three thoracic, and 

 none to eight abdominal ganglia. Beetles have, besides the central nervous system 

 noticed above, a somewhat complicated symjiathetic nervous system. 



The circulatory system of Coleoptera, like that of all insects, is not well differ- 

 entiated. A so-called heart extends along the dorsum, wliere it pushes the nearly 

 colorless blood toward the head ; here the blood is distributed in somewhat irregular 

 and usually ill-defined jiassages. Xeilher venous nor arterial system exists, in the full 

 sense of the terms, although in the mouth-iiarts and antenme of the larva of a water- 

 beetle {ILjdrophilus), when this interesting larva is seen alive under the mieroscojie, 

 the Idood can be observed circulating in definite channels. 



Respiration is by trachea> in the iinagos, and the respiratory movements consist of 

 alternate approximations and separations of the dorsal and ventral ])ortions of the 

 abdomen, causing renewal of the air in the trachea^. Besides the traclieic there are 

 generally tracheal 1)ull)S, or expansions of the tracheal which can be inflated at the 

 will of the insect, and are thouglit to be useful in flight, by lessening the specific 

 gravity of the insect. The stigmata (sometimes called spiracles), the external open- 

 ings of the trachea?, usually have more or less complete sieves to prevent dust from 

 entering the trachea', .and in some cases these sieves form, as in a water-beetle 

 (Di/tiscits), most beautiful objects for the microscope. Beetles have a pair of stigmata 

 on the mesothorax, on the metathorax, and on each of the first eight al)domiiial 

 segments ; their larva^ geiferally have one ]iair of thoracic, and eight pairs of abdominal 

 stigmata. Some aquatic larva? have only the two stigmata of the eighth abdominal seg- 

 ment, others respire wholly or partly by gills. 



The female sexual organs consist of numerous ovarian tubes on each side uniting 

 in various ways to form a ]iair of oviducts, which latter themselves unite to form 



