The Arachnida

THE ARACHNIDA THE arachnids are more easily recognized than defined. They have so many features in common with Limulus that some zoologists have classed Limulus in the Arachnida. The essential differences between the Xiphosurida and the Arachnida are in the feeding organs and the organs of respiration. The arachnids feed on liquids extracted from their prey, which are ingested by a pharyngeal sucking pump; the xiphosurids feed on solid food, which is ground up in a pro ventricular grist mill. The arachnids are terrestrial and breathe by means of lungs or tracheae; the xiphosurids, being aquatic, have abdominal gills, and theoretical attempts to derive the arachnid lungs from gills are not convincing. The most primitive of modern arachnids, the Palpigradi, are more generalized than Limulus. The Xiphosurida and the Arachnida, therefore, are two branches of the subphylum Chelicerata, but their common ancestors are not known. While there are paleontological reasons for believing that the xiphosurids and the trilobites had a com mon progenitor, the actual origin of the arachnids is obscure. How ever, as was noted in the last chapter, the pycnogonids have some surprisingly arachnoid characters. The scorpions have a superficial resemblance to the Eurypterida, but the scorpion, as compared with the Palpigradi, is not a primitive arachnid. However, it is not an ob ject of the present text to discuss theoretical arthropod phylogeny. The student may learn the essentials of arachnid anatomy from a study of the scorpion, the spiders, and a tick, which are the principal subjects of this chapter. 59 ARTHROPOD AN A TOMY THE SCORPION The scorpion in appearance (fig. 17 A) is a highly distinctive arachnid that could not possibly be mistaken for any other member of the class; the combination of large, chelate pedipalps with a sting at the end of a segmented tail alone proclaims the animal to be a true scorpion. The so-called whip scorpion (fig. 23 E) and the pseudoscorpion have no sting. About 600 species of scorpions are known, most of them two to four inches in length, but there is one only half an inch long, while the huge African Pandinus attains a length of seven inches. Scorpions are widely distributed throughout the tropical parts of the earth and in most of the warmer regions of the temperate zones; 22 species occur in the United States. For a general account of the habits and modes of life of scorpions the reader is referred to the article "Scorpion" by Petrunkevitch in the Encyclopaedia Britannica ( 1947). General Structure of a Scorpion A scorpion at first glance (fig. 17 A) appears to have an elongate oval body, supported on four pairs of legs, and a thick, jOinted tail bearing the sting at its extremity. The body, however, is divided into an unsegmented anterior part, which alone bears the appendages, and a larger segmented posterior part; the tail is a slender extension of the body, consisting of five segments, with the anus in the last segment. Morphologically considered, then, the trunk of the scorpion includes a prosoma, covered by an unsegmented plate, or carapace, and a segmented opisthosoma, or abdomen, which is differentiated into an anterior mesosoma, or preabdomen, and a posterior metasoma, or postabdomen, which is the tail. The prosoma bears the usual arachnid appendages, which are the chelicerae (A, B, ChI), the pedipalps (Pdp), and four pairs of legs (lL-4L); it therefore in cludes six primary postoral somites, and is thus comparable with the prosoma of Limulu8, except that it lacks the chilaria and the cor responding seventh segment. The seventh body segment of the scor pion, in fact, is known to be suppressed during embryonic develop ment, so that the firstse gment of the opisthosoma in the adult is the eighth. The mesosoma contains seven segments, which are segments VIII-XIV, and the tail has five segments (XV-XIX ), not including the terminal sting. The adult scorpion, therefore, has 18 postoral 60 THE ARACHNIDA (I) C�lI I B Fig. 17. Arachnida-Scorpionida. A, Chactas vanbenedeni Gervais, Chactidae (2 lateral eyes on each side), Colombia. B, Pandinus sp., Scorpionidae, Congo, ventral surface. C, same, dorsal (3 lateral eyes on each side ). D, Palaeophonus hunteri Pocock, Scottish Silurian scorpion, 35.5 mm. long (from Pocock, 1901 ). E, Pandinus sp., genital region of segment VIII, and pectines of segment IX, ventral. For explanation of lettering see pages 126-127. 61 ARTHROPOD ANATOMY segments in all, which is the maximum number of segments possessed by any other arachnid, but if we count the suppressed seventh seg ment, it has 19 segments. Studies by different writers on the correlation of the nerve centers of the scorpion with the body segmentation have given somewhat different results. McClendon (1904 ) described 20 pairs of neuro meres, or primary segmental ganglia, of which the first pair forms the brain, and the other 19 the ventral nerve cord. According to Buxton (1917), however, there are only 18 neuromeres in the nerve cord, which is one less than the number of body segments. Both Buxton and Petrunkevitch (1949 ), therefore, suggest that the first tail segment is a secondary subdivision of the last mesosomatic seg ment. Klistner (1940 ), on the other hand, re-examining the subject in species of several genera, finds that there are 19 pairs of primary ganglia formed in the postoral nervoussystem, of which the cheliceral ganglia unite with the brain, and the ganglia of segment VII disap pear along with the segment itself; in other words, there is at first a pair of ganglia for each of the primary 19 postoral segments. In the adult, however, the fivepostcheliceral ganglia of the prosoma and the firstfour persisting opisthosomatic ganglia unite in a large nerve mass, or sub oesophageal ganglion, lying in the prosoma, while the ganglia of segments XII to XVII remain separate, though the first two are displaced forward, and the ganglia of segments XVIII and XIX unite to form a double last ganglion lying in segment XVIII. The carapace of the prosoma (fig. 17 A, C, Cp ) is widest behind, somewhat narrowed anteriorly. On it are situated three groups of small, simple eyes, there being a pair of median eyes, and two groups of more anterior lateral eyes, each with from two to five eyes accord ing to the species. No arachnid has compound eyes. The front margin of the carapace projects as a free fold over the bases of the chelicerae (fig. 21 D). From the inner end of the under lamella, or doublure, of the fold, the membranous anterior wall of the body goes down ward to the base of a large median lobe (Lm), which is the labrum. The labrum lies between the bases of the pedipalps (B), but the chelicerae arise entirely dorsal to the labrum. At the base of the labrum is an irregular sclerotization of the body wall (D, Epst) representing the epistome of other arachnids, which is usually a hori zontal plate forming a bridge between the upper surfaces of the pedipalp coxae and supporting the labrum. Though the adult arach- 62 THE ARACHNIDA nid has no distinct head, the labrum, the epistome, and the eye bearing region of the carapace are derived from the cephalic lobe of the embryo. The lateral margins of the carapace are not extended beyond the leg bases (B), and a cross section of the prosoma of the scorpion (fig. 22 A), therefore, has quite a different shape from that of Limulus (fig. 7 A). The edges of the carapace are separated from the bases of the legs by narrow pleural folds of the integument (figs. 19 E, 22 A, PI). The ventral surface of the prosoma is occupied al most entirely by the coxae of the legs (fig. 17 B), there being only a small median sternal plate between the posterior two pairs of coxae. The fully segmented meso soma, or preabdomen, is much broader than the prosoma. On the dorsum (fig. 17 A) each of its seven seg ments is covered by a distinct tergal plate. On the undersurface (B) there are likewise seven segmental divisions, but the sternal plates are not all so simple or uniform in size as the tergal plates. The venter of the first segment (VIII ) is wedged between the anterior ends of the last leg coxae, where it is greatly reduced in size and has no connection with the tergum of its segment. On it, however, is situated the genital opening, or gonotreme, in each sex (B, E, Gtr), which is covered by a small plate or pair of plates forming an operculum (Opl), and is bordered behind by a transverse post genital fold (E, pgf ) of the integument. The second opisthosomatic sternum (B, IXS ) is a small quadrate plate bearing a pair of comb like appendages known as the pectines (Pee), which will be described in connection with the other appendages. This sternum also is sep arated from its tergum, except for narrow pleural folds that run along the posterior margins of the hind coxae. The following five mesosomatic sterna are broad plates united laterally with the cor responding terga by infolded pleural membranes (fig. 22 B, Pl) that allow a considerable dorsoventral expansion of the abdomen. The tergal and sternal margins of the last preabdominal segment, however, come together posteriorly and are united at the posterior angles of the segment. On the lateral areas of sterna X to XIII are oblique slits (fig. 17 B, Sp ), which are the apertures, or spiracles, of the internal respiratory organs known as book lungs, to be de scribed later. The five segments of the metasoma, or tail, are simple rings, the tergal and sternal arcs being entirely confluent.

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