984 



INSECTA. 



larva; of Gyr initial, in which they are arranged 

 along the sides of each segment as short stiff 

 bristles. This form of branchiae is also said to 

 exist in one solitary species of Lepidoptera, 

 Hydrocampa stratiotatu* and perhaps, also, 

 in the other species of the same genus, in 

 which branchiae of this form exist in the neigh* 

 bourhood of false spiracles. 



The second kind of branchiae exists in the 

 form of flat, oval, or lanceolate plates, extended 

 from the sides of each segment of the abdo- 

 men, where spiracles afterwards exist in the 

 perfect insect. In some instances, as in the 

 Agrionidte, these plates exist only at the extre- 

 mity of the abdomen. In others, as in the 

 Ephemerida, they exist both at the sides and at 

 the extremity of the body. This form of 

 branchiae is found only in the Neuroptera and 

 Trichoptera. In many of the latter instances 

 these parts possess also the additional function 

 of being the chief locomotive organs of the 

 insect, and remind us strongly of the branchi- 

 form organs of locomotion in the post-abdo- 

 men of many Crustacea. 



An anomalous insect, recently discovered by 

 Mr. Hogg as a constant inhabitant of the river- 

 sponge, and an account of which was read by 

 Mr. Westwood at a meeting of the Entomolo- 

 gical Society on the 3rd of December, 1838, 

 possesses a third and most remarkable descrip- 

 tion of branchia. This insect, which was re- 

 ferred by Mr. Westwood to the order Neurop- 

 tera, genus Acentropus, STEPH. very much 

 resembles the larva of a neuropterous species, 

 and has filiform branchiae extended from the 

 sides of the abdomen, which are distinctly or- 

 ticuluted, and apparently five-jointed, Mr. 

 Westwood informs us that he has distinctly 

 traced tracheae into the branchiae, and that the 

 open extremity of each vessel protrudes from 

 the tip of the branchiae, so that in this respect 

 these organs resemble elongated spiracles. 



Very few of those larvae or pupae that pos- 

 sess branchiae have any lateral spiracles, ex- 

 cepting the Culicidte. In some of these, as in 

 the common Gnat, both larvae and pupae breathe 

 by means of large tracheal vessels, extended 

 outwards to some distance from the head and 

 thorax, while the body is also furnished with 

 filamentous branchiae. The larva of Chiro- 

 nomus, remarkable for its blood-red colour, and 

 which breathes through tubes, is furnished in 

 its pupa state with branchiae f at a part cor- 

 responding to that at which the first spiracles 

 are to appear in the perfect insect. The true 

 Libelluuda have neither lateral nor anal bran- 

 chiae, but, according to Suckow and others, 

 breathe by means of branchiae in the colon. 

 But in these insects, as in the Ephemerida 

 with lateral branchiae, the acts of respiration are 

 also those of progression ; since, although the 

 imbibition and expulsion of water at the anal 

 extremity are regular and constant, even when 

 the insect is remaining perfectly quiet, they are 

 increased both in number and force at every 

 act of locomotion, and carry the body forward 



* Id. vol. i, pi. xxxvii. 

 t Burnaeister, p. 167. 



by darts or sudden impulses. Tlte water is 

 received at the anal orifice by an inspiratory or 

 sucking action, as proved by the circumstance 

 that small particles of substances floating in 

 the water are drawn in with the stream at the 

 anus at each inspiration, and again expelled 

 from it when the water is ejected, and this oc- 

 curs with the greatest regularity. The cavity 

 into which the water is received is a cloaca 

 distinct from the proper alimentary canal, 

 analogous to the respiratory cloaca of the Ho- 

 lothuria and other lower invertebrata. 



These external organs of respiration all com- 

 municate with elongated tracheae, from which 

 are distributed other branches over the internal 

 structures. In the larvae of all insects these 

 internal respiratory organs are simply ramified 

 tubes, but in perfect insects, and more parti- 

 cularly in volant species, these tubes are di- 

 lated into an immense number of minute 

 vesicles, which not only allow of the most ex- 

 tensive respiration, but also render the body 

 lighter, by enabling the insect to alter its spe- 

 cific gravity during flight. In all insects the 

 vesicles are only dilated tracheae, the structure 

 of which is the same throughout the class. In 

 the larva of the Sphinx and other insects the 

 tracheal vessels consist of two elongated tubes, 

 extended one on each side of the body, and 

 which on their external side communicate by 

 very short tubes with the spiracles, and on their 

 opposite, towards the middle line of the body, 

 give off near each spiracle a large tuft of ves- 

 sels, about twelve in number, which extending 

 inwards are distributed over the different 

 organs within the body. The chief of these 

 are distributed to the alimentary canal, while 

 others pass upwards among the muscles, and 

 ramify most extensively between them, and are 

 given in great abundance to the dorsal vessel. 

 In every segment other branches are extended 

 to the median line above the vessel, and anas- 

 tomose with corresponding branches from the 

 opposite side of the segment. In the head one 

 large branch passes forwards from the pro- 

 thoracic spiracle, and anastomoses with its 

 fellow from the opposite side above the oeso- 

 phagus, behind the brain, and the two thus 

 united then give off four branches, which 

 passing forwards over the brain give off branches 

 that ramify on the surface, and even in the 

 substance of that organ and of the optic 

 nerves. Other branches are given to the 

 muscles of the head, the future antennae, and 

 the organs of manducation. On the under 

 surface of the body the tracheal vessels dis- 

 tribute themselves among the muscles, as on 

 the dorsal surface, some passing beneath the 

 nervous cord to anastomose with those from the 

 opposite side, while others are distributed to, 

 and ramify extensively over the surface of each 

 gangliated portion of the cord, giving off the most 

 minute branches which penetrate the very sub- 

 stance of the ganglia (fig. 415, i, i), and also 

 that of the nerves themselves, along which other 

 minute branches are extended, so that even the 

 most important and delicate organs of the 

 body are plentifully supplied with tracheal 

 vessels. Other branches of tracheae also pass 



