2:3 



INSECT A. 



INSECTA. 



Z51 



At its maximum of development it consists of four pieces above 

 and eight below, to which Audouin applies the names Prsescutum, 

 Scutum, Scutellum, and Postscutellum, to the upper pieces, or tergum ; 

 and Paraptera, Sternum, Episterna, Epimera, and Medifurca, to the 

 mesosternum. The metathorax, as it bears the posterior wings, is 

 well developed in those insects which possess them, but where they 

 are wanting, as in the order Diptera, it is of small size. Its upper 

 surface is called Metanotum, and the under surface Metasternum. 

 When perfect it contains the same number of parts as the mesothorax. 

 To this segment are attached the posterior pair of legs. 



The various parts of the thorax will perhaps be better understood 

 by their being placed iu a tabular form, thus 



Thorax. ( 



Prothorax. ( 



Mesothorax. 



Tergum, or 

 Pronotum. 



Pectus, or 

 Prosternum. 



Metathorax. 



C Prsescutum. 

 I Scutum. 

 | Scutellum. 

 l_ Postscutellum. 

 I Sternum. 

 < Episterna. 

 \_ Epimera. 

 Furca, called Aitefurca. 



("Pnescutum. 

 Tergum, or I Scutum. 

 Mesonotum. | Scutellum. 



(. Postscutellum. 

 f Paraptera. 



Pectus, or I Sternum. 

 Mesosternum. | Episterna. 

 l_ Epimera. 

 Furca, called Medifurca. 



fPrajscutum. 

 Tergum, or J Scutum. 

 Metanotum. | Scutellum. 



L Postscutellum. 



T Paraptera. 



I Sternum. 



Pectus, or 



Metasternum. ] Episterna. 

 I Epimera. 

 Furca, called Postfurca. 



From the thorax we are naturally led to the wings and legs of 

 insects. 



The greater portion of the insect tribe possess four wings ; some 

 however only possess two, and others are quite destitute. These 

 organs consist of two membranes applied closely together, and inclosing 

 numerous nervures or hollow tubes which contain trachea;. 



The various descriptions of wings may be described under the 

 following heads Elytra, Tegmina, Hemelytra, and Halteres. The term 

 elytra is applied to the anterior wings. " When they are without 

 nervures and uniformly of a thicker and harder substance than mem- 

 brane," they are peculiar to the Coleoptera. [COLEOPTERA.] 



Tegmina is the name applied to the upper organs of flight when of 

 a uniform coriaceous texture, and furnished with nervures as in the 

 Orthoptera. [ORTHOFTEHA.] 



Hemelytra, the upper organs of flight when they are coriaceous at 

 the base and membranous at the apex, as in the ffemiptera. 

 [HEMIITERA.] 



The Halteres are two minute organs situated behind the wings of 

 Dipterous Insects, and supposed to represent the posterior wings ; they 

 consist of a slender stalk with a round or oval knob at the extremity. 

 [DIPTERA.] 



The Legs in true insects are invariably six in number, but in certain 

 butterflies the anterior pair are minute. Each leg consists of a Coxa, 

 Trochanter, Femur, Tibia, and Tarsus, all of which parts are figured 

 and described in the article COLEOPTERA. 



The Abdomen. Although the nine segments which compose the 

 abdomen are generally distinct in larvoo, we seldom find more than 

 seven or eight visible joints iu the perfect insect, the remaining one 

 or two being generally hidden, and in fact converted into parts of the 

 organs of generation. The number of segments to the abdomen 

 sometimes differs in the males and females of the same insect, as in 

 the Aculeate Jfymcnoptera. As these segments in the perfect insect 

 bear no organs of locomotion, they are of a more simple structure 

 than those of the thorax, consisting chiefly of an upper plate called 

 the Dorsum, and an under plate called the Venter. 



The substance of the abdominal segments is almost invariably less 

 hard and more flexible thau that of the head and thorax. 



In the Coleoptera and llemiptera, where the upper parts are pro- 

 tected by elytra, or hemelytra, they are softer than on the under 

 mrface which is exposed. In certain species however where the 

 elytra do not cover the abdomen they are of the same substance 

 throughout, as for instance in the Staphylinidce and several minor 

 groups of Coleopterous Insects. The articulation of the abdomen to 

 the thorax offers some curious modifications, some of which are con- 

 utant throughout whole groups, and hence afford distinguishing 

 characters. When the abdomen is closely applied to the thorax it is 

 termed sessile ; and when the first segment, or more, is narrow and 

 elongated, and forms a kind of stalk, it is termed petiolate. 



The abdomen is often furnished with appendages at its extremity ; 

 thus in the Earwig (in which Mr. Westwood discovered one more 

 than the usual number of segments) there is a pair of forceps which 

 serve as weapons of defence, and in the male sex of Panorpa, where 

 the apex of the abdomen is considerably elongated, there is also a 

 pair of forceps. In the Dragon-Flies there are small flattened append- 

 ages, and likewise in the Siaphylinidce, which are called Stylets. 

 Indeed the various kinds of appendages are too numerous to be here 

 described, but are noticed in the accounts of the various groups of 

 insects contained in this work. The modifications of the ovipositor 

 are likewise noticed where they occur in the different groups. When 

 it is of a long and compressed form it is termed ensate ; and when 

 it consists of several tubes retractile within each other, like the pieces 

 of a telescope, it is called telescopiform. The term aculeiform is 

 applied to this organ in the Hyrnenopterous Insects. 



We now come to the internal anatomy of insects. 



The Digestive System of insects is well developed, and consists of 

 an intestinal canal, in which a crop, gizzard, stomach, and small 

 intestine are generally distinct ; but, as in the higher orders of animals, 

 these parts vary according to the naturgjf the food. 



Fig. 28. 



Fig. 28, Intestinal canal of Cmndela campeitrit : a, oesophagus ; b, crop ; 

 f, gizzard ; d, stomach ; e, e, hepatic vessels ; /, small intestine. 



In a predaceouB beetle (Cicindda) the intestinal canal passes nearly 

 straight through the body, the oesophagus dilates into a wide crop, 

 which is succeeded by a minute gizzard, and then by the stomach, 

 which, as well as the crop, is covered by numerous minute follicles ; 

 at the pyloric extremity of the stomach the biliary ducts pour their 

 secretion into the cavity through four orifices, which are situated two 

 on each side. The small intestine is short, and opens into a wide 

 colon which terminates the canal. 



In a vegetable-feeding insect the stomach is very voluminous and 

 much convoluted, and the biliary ducts are proportionately long. In 

 the common cockchafer these ducts have the secreting surface 

 increased by innumerable minute secse. 



The salivary glands are distinct in many insects. 



The circulation of the blood in insects is carried on in part by 

 means of distinct vessels, and in part by channels excavated in the 

 tissues. Its central organ is the dorsal vessel, which is segmentally 

 divided ; the compartments are separated by valves, which do not 

 allow the blood to pass in any other direction than from behind 

 forward. This segmental division however in the perfect insect does 

 not extend into the thorax, the dorsal vessel in that region being 

 converted into an aortic trunk, which carries the blood onwards, that 

 it may be distributed to the head and thoracic appendages. From 

 these it returns backwards along the limbs and body to re-enter the 

 dorsal vessel, either by veins which open into its several chambers, 

 or by larger vessels that collect the whole to convey it into the 

 posterior chamber. In its course however it is brought into very 

 close relation with the air that is conveyed through the whole interior 

 of the body by the complex tracheal apparatus ; for it appears from 

 recent observations that the blood not only bathes the exterior of the 

 air tubes, but moves through that space between the outer and inner 

 membranes in which a spiral filament winds (as in the spiral vessels 

 of plants) to keep them from being closed by lateral pressure. 



These air-tubes form a complex system, which is distributed with 



