306 NATURAL HISTORY OF ARTHROPODS. 



tious in carnivorous beetles and more convolutions in herbivorous ones, may be 

 designated as the ventricle. The posterior end of the ventricle is marked by the 

 insertion of four or six long tubes, the malpighian vessels, which are urine-excreting 

 organs. Beyond the insertion of the malpighian vessels, and ending at the anus, 

 is the intestine, properly speaking, often showing a differentiation into small intestine, 

 caecum, and rectum. The malpighian tubes are usually a number of times as long as 

 the insect itself, and are coiled about in the abdomen of the beetle in a way that makes 

 them very difficult to disentangle without breaking. Glands open into various portions 

 of the digestive tract, among them rectal glands into the rectum. Carabidre, Dytis- 

 cidas, and some other beetles have glands opening on each side of the anus that secrete 

 acid or strongly odorous fluids which are used by these insects to defend themselves. 

 In the bombardier-beetle (Brachimis) the secretion of these anal glands is partly 

 gaseous, or becomes aeriform immediately after its discharge. 



The nervous system consists of a supra-oesophageal ganglion, from which originate 

 the antennal and optic nerves ; an infra-cesophageal ganglion which sends nerves to 

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

 commissures, just beneath the digestive tract. During the growth of the larvse these 

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

 various ways, so that, in the imagos, Eduard Brandt has characterized four types of the 

 nervous system, as follows : first, a system with a supra-oesophageal ganglion and a 

 large central nerve-mass in the thorax, of which mass the forward end is the infra- 

 cesophageal ganglion ; second, 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 abdominal ganglia ; fourth, a system with two cephalic, three thoracic, and 

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

 noticed above, a somewhat complicated sympathetic 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, where it pushes the nearly 

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

 and usually ill-defined passages. Neither venous nor arterial system exists, in the full 

 sense of the terms, although in the mouth-parts and antennae of the larva of a water- 

 beetle (Jlt/drop/iilus), when this interesting larva is seen alive under the microscope, 

 the blood can be observed circulating in definite channels. 



Respiration is by tracheae in the imagos, and the respiratory movements consist of 

 alternate approximations and separations of the dorsal and ventral portions of the 

 abdomen, causing renewal of the air in the tracheae. Besides the tracheae there are 

 generally tracheal bulbs, or expansions of the tracheae, which can be inflated at the 

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

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

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

 entering the tracheae, and in some cases these sieves form, as in a water-beetle 

 (Dytiscus), 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 abdominal 

 segments ; their larva? generally have one pair of thoracic, and eight pairs of abdominal 

 stigmata. Some aquatic larvae 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 pair of oviducts, which latter themselves unite to form 



