314 ARACHNIDES. 



ated or ramified tracheae *, that only receive air through two stigmata ; 

 in the absence of an organ of circulation!; and in the number of 

 their eyes, which is but from two to four ;{:. The want of sufficiently 

 general anatomical observations, has prevented the limits of this 

 order from being rigorously determined. Some of these Arachnides, 

 the Pycnogonides for instance, exhibit no stigmata ; their mode of 

 respiration is unknown. 



The Trachearise are very naturally divided into those which are 

 furnished Avith chelicerae, terminated by two fingers, one of which is 

 moveable, or by one that is equally so ; and into those where these 

 organs are replaced by simple laminae, or lancets, which with the 



* The tracheae are vessels which receive the aerial fluid and distribute it to every 

 part of the interior of the body, and thus remedy the want of ciixulation. They are 

 of two kinds. Those that are tubular or elastic are formed of three membranes, the 

 intermediate of which is composed of a cartilaginous elastic filament spirally con- 

 torted ; the two others are cellular. The resicular trachese consist of but two mem- 

 branes of the latter description. They are a kind of pneumatic pouches susceptible 

 of being inflated and depressed. Aquatic Insects, and others that are aerial, are 

 deprived of them. They communicate with each other by tubular trachea;. In 

 several of the Orthoptera, where they are well developed, cartilaginous arches, 

 formed by appendages of the inferior semi-annuli of the abdomen, give points of 

 attachment to the muscles which form them. The branchiffi are divided into two 

 principal trunks which extend longitudinally throughout the body, one on each side, 

 receiving air through lateral openings or stigmata, and then throwing off numerous 

 branches and twigs which distribute it. In several Insects, however, there are two 

 other trunks more or less long, situated between the two preceding ones, and com- 

 municating with them. M. Marcel de Serres distinguishes them by the term pulmo- 

 nary trachece : the others he calls arterial trachea;. He also distinguishes two sorts 

 of stigmata : one kind, or the ordinary stigmata, simple, and consisting of two 

 membranous lips, furnished with transverse strise or fibres, and opening merely by 

 contraction ; the others, which he calls tremaeres, are formed of one or two (usually 

 two) horny, moveable pieces, opening and closing like shutters. De Geer — Descript., 

 Gryllus migratorius — compares them to eye-lids. They are peculiar to certain 

 Orthoptera, and their position shows them to be the stigmata of the mesothorax. 

 M. Leon Dufour — Ann. des Sc. Nat., May 1826 — has given excellent figures of 

 these various kinds of stigmata, but without employing the names of the preceding 

 authors. It would appear from his description of the abdominal stigmata, that they 

 have the characters of the tremaeres, while those which he afterwards describes as 

 different, are the ordinary stigmata. Our own opinion is that these differences are 

 mere simple modifications of the lips. Reaumur, Mem., I, iv, 16, has figured a 

 stigma of this latter kind, where the lips have an internal border, which, from all 

 appearances, must be corneous. By supposing them to be almost entirely of this 

 nature, we have the trema^re of M. de Serres. Certain aquatic larvae have a pecu- 

 liar respiratory apparatus, of which we shall speak hereafter. 



■f- The presence of tracheae excludes a complete circulation, that is to say, the 

 distribution of the blood to the different parts of the body, and its return from the 

 organs of respiration to the heart. Thus, although some vessels have recently been 

 discovered in certain Insects — Phasmse — and, although they may possibly exist in 

 various Arachnides Tracheariae, it does not exclude them from the general system. 

 M. M. de Serres has observed that the intestinal tube of the Phalangium gives off 

 numerous caeca or vermiform appendages, which seem to have some analogy with the 

 hepatic vessels, and that the tracheae ramify over them ad infinitum. 



X According to Miiller the Hydrachna umbrata has six eyes: bat may this not 

 have arisen from an optical illusion or some mistake.'' 



