Savory - Arachnida 1977

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·················--~ 96 II. DE ARACHNIDIS distinguished, the same mysterious pectines and the same terminal sting. These arc not characteristics of a primitive animal, so that one is forced to imagine a hypothetical ancestor, a truly primitiye scorpion from which the Silurian forms were descended. This imaginary protoscorpion can more easily be imagined to be the ancestor of the Eurypterida than can any known extinct scorpion. Nevertheless, the hypothesis is not one that calls for immediate recognition . .\iigration from the land to the water is not to bF readilv accFpted, nor does any Eurypterid a scorpion-like anccstr):. In the absencF of any fossil scorpion earlier than tlw Silurian the speculation cannot usefully be pushed further. There is a frustrating lack of evidrncc from the ensuing Dcvonian epoch, with very few arachnid fossils. This period was the first to have left t'vidence of great changes in the environment. The seas continued to be overcrowded; Pterygotus was prominent, and fishes were so numerous that the time is often known as the Age of Fishes. On land, invertebrate life dcnJoped: there were Ephemeroptera 'Nith a wing span of 13 cm, and an intelligr·nt observer, ft1llowing the course of foretelling the future by recollection of the past, v.'ould han~ prophesied a ma~s invasion of the land. It came in the Carboniferous, with nothing less than an arachnid explosion. A warm, moist atmosphere favoured a territorial existence; plant life was luxuriant insects and myriapods were accornpanied by uearly all the orders of ,\raehnida that we recognize today. For many marine animals life in thr: sea was becoming less and less tolera hle, and for the Enrypterida at any rate it was about to pass the acceptable limit. They were l(1r extinction, and did. in fact, become extinct in the succeeding Pcrmian epoch. The temptation to go ashore in search of better conditions must ha,·e been great; and, as Goetbc l1as "Animals are attempting the impossible, and often it". In the end their achie,·ements resulted in the creation of some 16 orders, the geological distribution of which is displayed in the following, familiar table. Such a fragmentary record does not give as much help as be C'xpccted. It shows that most of the orders \V ere in existence during the Carboniferous era, and that they largely e\'olnd along different liues, as a result cf which five of them disappeared and 12 surviHd. The most significant fact that emergt's from this vie\\· of the palaeontology of Arachnida is the conviction that they provide a good example of that general parallelism in evolutionary progress which is such ;1 conspicuous feature of evolution as a whole. The idea of the evolution of early scorpions from contemporary Schizomida Araneae Ku>tarachnae Anthracomarti Haptopoda :\rchitarbi Opilioncs Cyphophtbalmi Acari Ricinulei Pseucloscorpiones Solifugae .~ ,_ ~ Cl "" X X ll. PHYLOGENY: EVOLUTION PRl:VIARY Y:t :;1 c:: :- .~ c:: (, s ~ .~ > "" § 0 ,_ .I:; 0 ,:; ,_ i) 8 ;.. ;:; c. "" ..._, :;. "' v c. SECO:-IDARY (.; \) :!l ·~ ·~ ,_ ~ ;:; ~ j ;:: .:3 "' j :..., -, Cl X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X TlcRTIARY X X X X I X X X X X X X X X X X X X X X X X X X X X Eurypterida seems to ha\·e been accepted so \vhole-heartedlv that further im·estigation has been ; yet the taxonomic isolation of the order Scorpiones should have indicated the need for some intnpretation. The scorpions cannot be regarded as the ancestors of the other orders, and this suggests that during the late Devonian or early Carboni!Crous many other occupants of the iucreasingly unpleasar{t sea.mu.st b~,-e foll?we~ly1e c.:;:amplc oftheir predecessors and repeated thetr lnstonc landmg. 1 here 1s no reason to suppose that all these were of the same species, or e\·en of the same family; more probablv thev were of different types, and the terrestrial Arachnida that e\·olnd. fron~ them were different in consequence. \Ve call them different orders. Fossil scorpions seem to arrange thcmsehes in nine super-families, only one of which can claim relationship with the living scorpions of the p:esen~. One may say, in broad outline, that ancestral scorpions of mne kmds made the experiment and that only one succeeded. Let the same be applied to the later invasion, and it may be said that 16 explorers set foot ashore and ll of them founded p~rmanent orders. Survival rate had risen from ll to 69%. One wonders why. 97
98 II. DE ARACHNIDIS An obvious assumption is that the land was in a condition more favourable to sustain life than it had been at the earlier period, and that, correspondingly, the new arrivals were better fitted to exploit the improvements. And how better fitted? If we are right in guessing that they had come not from the depths of the ocean, but from the shallower or even from the littoral zone, they would already have become accustomed to exposure during low tides, when the use of atmospheric oxygen was essential. This is most likely to be possible for a small animal, seeking shelter under rocks, stones or sea weeds. If the relatively large surface of a small animal can be kept moist, the moisture may dissolve enough oxygen to avert catastrophe. The danger to the pioneers came, therefore, not so much from the difficulty of respiration as from the risk of desiccation, as formidable a threat then as it is today. Arachnida and other Arthropoda can avoid its worst effects because of a layer of wax in the cuticle. In marine organisms such a layer may have controlled the passage of water by osmosis in and out of the body. This would have been particularly valuable to esturine species, exposed to recurrent changes in the salinity ofthe surrounding water. This might be claimed as an example ofpreselection for survival in a new environment, where the immigrants would swell the ranks of the animals, still in the majority, that are known collectively as the cryptozoa. Nor should the possibility be neglected that some of the survivors did not leave the sea voluntarily, if one may use the term, but were carried ashore by tidal waves or in spray during an onshore gale. Cataclasms of this kind have been adduced by other zoologists to explain some of the peculiarities of evolution and distribution. Small organisms, thus illtreated, might survive the experience, while many others must have failed to do so. It appears that 16 kinds succeeded. These products of speculation and imagination may well be followed by, and indeed contrasted with, more orthodox attempts to trace the evolutionary steps by which the orders have become as distinguishable as they are today. The Arachnida which combine the greatest number of primitive features are the Palpigradi; it is also undoubted that the greatest number of specialized features is nine or ten, the total found in the Ricinulei. The many structural differences between these two extremes guide us in our search, and the studies of Petrunkevitch ( 1949) suggested one way of attempting this reconstruction of the past. There is no arachnid in which all the prosomatic tergites are separate, and in nearly all orders the prosoma is covered by a uniform carapace. However, in the three orders Schizomida, Solifugae and Palpigradi there is a large propeltidium in front, with smaller plates, the mesopeltidium 11. PHYLOGENY: EVOLUTION 99 and metapeltidium, behind it. This suggests that fusion of the tergites has proceeded backwards from the head. On the ventral surface the sternites ha,·e partly fused in some orders to form a large sternum, or ha,·e disappeared. These changes are associated with the monment of the mouth and the approach of the coxae towards the centre. There is always a labium or lower lip, the sternite of the second somite, and in the Schizomida, Uropygi and Palpigradi there are persistent sternites behind it. The backward movement of the mouth to a position between the pedipalpi, which can be clearly witnessed during indi,·idual dewlopment, is common to all Arachnida. In the extinct orders Haptopoda and Anthracomarti, as in the living king crabs, it is between the coxae of the first pair oflegs. The coxae of the pedipalpi and legs take shares, in varying degrees, in the mastication of the prey, so that their approach to each other is not surprising, and in Opiliones and Solifugae no remnant of sternites is usually visible. The last, fundamentally the sixth, sternite sometimes persists between the fourth coxae, a position which, in Solifugae, is occupied by the first sternite of the opisthosoma. In the opisthosoma changes have occurred in the first somite and some of the posterior somites. The first tergite may be lost, as in recent scorpions, or fused with the carapace, as in the Opiliones. In the orders grouped as the Caulogastra it is constricted or reduced in circumference, tc form the pedicel, seen in its primitive form in the Palpigradi. The last three somites show different changes. They may be rather similarly constricted to fcrm the narrow region known as the pygidium, to be seen in Uropygi, Schizomida and Ricinulei; but these regions are not strictly homologous, for in the U ropygi the pygidium represents somites 10, 11 and 12, in the Schizomida 9, 10 and 11, and in the Ricinulei 7, 8 and 9, or, perhaps, 8, 9 and 10. The last stage in the process is seen in spiders, where the anal tubercle represents from one to three persistent tergites, their sternites having vanished. In spiders other than Liphistiidae only five opisthosomatic somites remain, and in mites there is often no trace either of segmentation or of anal tubercle. This reduction of the opisthosoma has been carried to its extreme limit in the Pycnogonida, a characteristic which well defines the relationship and the difference between the Pycnogonida and the Arachnida. These evolutionary changes in the construction of the arachnid body may therefore be summarized by saying that in the prosoma there has been a gradual fusion of the tergites proceeding from before backwards, and in the opisthosoma a loss or fusion of somites proceeding from behind forward. It may be added that these changes appear to have taken
Page 1 and 2:
Arachnida THEODORE SAVORY 2nd editi
Page 3 and 4: Contents Preface Acknowledgements .
Page 5 and 6: MAIAE CARISSIMAE UXORI SECUNDAE LIB
Page 7 and 8: 1 The Phylum Arthropoda The phylum
Page 9 and 10: 6 I PROLEGOMENA The comparison impl
Page 11 and 12: 10 I. PROLEGOME);A one time so rare
Page 13 and 14: 14 I. PROLEGOMENA The most striking
Page 15 and 16: 18 U. DE ARACHNIDIS it, move to a f
Page 17 and 18: 3. MORPHOLOGY: EXTERNAL APPEARANCE
Page 19 and 20: 26 II. DE ARACHNIDIS 3. MORPHOLOGY;
Page 21 and 22: 30 II. DE ARACHNIDIS yet arachnolog
Page 23 and 24: 34 II. DE ARACHNlDIS curved. It oft
Page 25 and 26: 38 II. DE ARACHNIDIS 4. PHYSIOLOGY:
Page 27 and 28: 42 II. DE ARACHNIDIS fibres run upw
Page 29 and 30: 46 II. DE ARACH~IDIS As a rule the
Page 31 and 32: 50 11. DE ARACHNIDIS (iii) Inversio
Page 33 and 34: 54 II. DE ARACHNIDIS FIG. 21. Newly
Page 35 and 36: 58 II. DE ARACHNIDIS Pseudoscorpion
Page 37 and 38: 62 11. DE ARACHNIDIS of the exoskel
Page 39 and 40: 66 II. DE ARACHNIDIS Sometimes the
Page 41 and 42: 70 II. DE ARACHNIDIS using a scanni
Page 43 and 44: 74 II. DE ARACHNIDIS constitutes th
Page 45 and 46: 78 II. DE ARACHNIDIS think of Arach
Page 47 and 48: 82 II. DE ARACH::-.;IDIS declined t
Page 49 and 50: 86 II. DE ARACHNIDIS .Man is the on
Page 51 and 52: 90 11. DE ARACHNIDIS Autecology con
Page 53: 11. PHYLOGENY: EVOLUTION 95 11 Phyl
Page 57 and 58: 102 II. DE ARACHNIDIS start preserv
Page 59 and 60: 106 II. DE ARACHNIDIS Here are thre
Page 61 and 62: 110 11. DE ARACHNIDIS Araneae, and
Page 63 and 64: 13 The Order Scorpiones [Scorpionid
Page 65 and 66: 118 Ill. PROLES ARACHNES 13. THE OR
Page 67 and 68: 122 III. PROLES ARACHNES CLASSIFICA
Page 69 and 70: 126 III. PROLES ARACHNES 14. THE OR
Page 71 and 72: 130 Ill. PROLES ARACHXES The lowly
Page 73 and 74: 134 Ill. PROLES ARACHNES to be seen
Page 75 and 76: 16. THE ORDER SCHIZOMIDA 139 16 The
Page 77 and 78: 142 Ill. PROLES ARACHKES power, an
Page 79 and 80: 146 III. PROLES ARACHNES DISTRIBUTI
Page 81 and 82: 150 III. PROLES ARACHNES chelicerae
Page 83 and 84: 154 Ill. PROLES ARACHNES FIG. 53. S
Page 85 and 86: 158 Ill. PROLES ARACHNES (vi) Lobed
Page 87 and 88: 162 III. PROLES ARACHNES is found i
Page 89 and 90: 166 III. PROLES ARACHNES 4 (3) Chel
Page 91 and 92: 20. THE ORDER TRIGONOTARBI 171 20 T
Page 93 and 94: 174 Ill. PROLES ARACHNES 22 The Ord
Page 95 and 96: 178 III. PROLES ARACHNES The family
Page 97 and 98: 182 III. PROLES ARACHNES are above
Page 99 and 100: 186 Ill. PROLES ARACHNES opisthosom
Page 101 and 102: 190 Ill. PROLES ARACHNES Sub-order
Page 103 and 104: 194 III. PROLES ARACIINES 25. THE O
Page 105 and 106:
26 The Order Acari 26. THE ORDER AC
Page 107 and 108:
202 III. PROLES ARACHNES Tetrastigm
Page 109 and 110:
206 Ill. PROLES ARACHNES 26. THE'.
Page 111 and 112:
210 Ill. PROLES ARACHNES ( 1) Notos
Page 113 and 114:
214 Ill. PROLES ARACHNES The sevent
Page 115 and 116:
218 Ill. PROLES ARACHNES family, Ho
Page 117 and 118:
222 Ill. PROLES ARACHNES posterior
Page 119 and 120:
226 III. PROLES ARACHNES 28. THE OR
Page 121 and 122:
230 Ill. PROLES ARACHNES From the f
Page 123 and 124:
234 Ill. PROLES ARACHNES 29. THE OR
Page 125 and 126:
238 Ill. PROLES ARACHNES The chelic
Page 127 and 128:
242 IlL PROLES ARACHNES 29. THE ORD
Page 129 and 130:
30 Economic Arachnology The Arachni
Page 131 and 132:
250 IV. DE ARACHNOLOGIA Although th
Page 133 and 134:
··~~- -..... 254 IV. DE ARACHNOLO
Page 135 and 136:
31 Historical Arachnology The histo
Page 137 and 138:
262 IV. DE ARACHNOLOGIA spiders qui
Page 139 and 140:
266 IV. DE ARACHNOLOGIA little was
Page 141 and 142:
32 Practical Arachnology The practi
Page 143 and 144:
274 IV. DE ARACHNOLOGIA hard. In th
Page 145 and 146:
278 IV. DE ARACHNOLOGIA EXCURSUS A
Page 147 and 148:
33 Chemical Arachnology In the stud
Page 149 and 150:
286 IV. DE ARACHNOLOGIA not very pl
Page 151 and 152:
290 IV. DE ARACHNOLOGIA seventeenth
Page 153 and 154:
294 IV. DE ARACHNOLOGIA A new aspec
Page 155 and 156:
298 IV. DE ARACHNOLOGIA For more th
Page 157 and 158:
36 The Spider's Web The webs that s
Page 159 and 160:
306 V. HETEROGRAPHIA a number ofver
Page 161 and 162:
310 V. HETEROGRAPHIA dramatically p
Page 163 and 164:
314 ( i) ( ii) ( iii) (iv) (v) V. H
Page 165 and 166:
318 V. HETEROGRAPHIA this may be ad
Page 167 and 168:
322 V. HETEROGRAPHlA .1\Jenge in 18
Page 169 and 170:
326 VI. EPILEGO:VIENA Vellard, J.,
Page 171 and 172:
i 330 Vl. EPILEGO~iENA Lamoral, B.
Page 173 and 174:
334 VI. EPILEGOMENA Ill. Bibliograp
Page 175 and 176:
INDEX ANIMALIUM 339 Index Animalium
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Savory - Arachnida 1977

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