M.vK(.u 1. 1901.] 



KNOWLEDGE. 



53 



Distinguished from the family of Isotoma by the 

 relatively lai-ge protliorax. the frequent reduction of the 

 ventral tube to a mere tubercle, and the presence of the 

 spring ^if one be present) ou the fourth segment of the 

 hind-body, the Poduridie form the lowest of the three 

 families of Spring-tails. They arc lowly not because 

 they are primitive but because they are degenerate. 

 Among the genera of this family the spring can be traced 

 through all stages of degradation until it vanishes al- 

 together, and we have a parados — like many another 

 equally instructive in zoology — a Spring-tail without a 

 spring. 



The genus Achorules, however, is characterised by the 

 presence of a well-developed spring, and its most abund- 

 ant species. A. vialicus. Tullberg. seems to be. in certain 

 places at least, the commonest of all the insects of our 

 coasts. Like Isotoma pahistri-i mentioned above, it also 

 abounds in many inland localities. Colonies may be 

 found sheltering in moss, or disporting themselves on 

 the top of a wall, or cooling themselves in a marshy 

 hollow, or luxuriating in a rotten turnip. But they 

 may often be found in numbers among the cast-up sea- 

 weed, and sometimes on the sea-sands they occur in vast 

 myriads. At Aberladv. near Edinburgh, in September, 

 1896. Mr. Evans found'these Spring-tails, 2000 or 3000 to 

 the square foot, for several hundred yards along the shore 

 of the Bay. The foreign range of the species stamps it 

 as a dominant race. It is known to occur from Siberia 

 to California, from Spitsbergen and Greenland to Tierra 

 del Fuego. 



Fig. 4. — Seni/lla humicola. magnified 2U times, a, foot- 

 claw ; J, tooth and tip of spring, magnified 120 times ; c, tooth 

 and tip of spring of j. maritima, magnified 120 times. 



Two rare kinds of marine Spring-tails belong to the 

 genus Xenylla, which is distingixished from Achorutes 

 by the presence of only one claw on the foot instead of 

 two. These insects are deep blue-black in colour. One 

 of them, A', liumicola. O. Fab. (Fig. 4), until its recent 

 discovery on the North German and Scottish coasts, 

 was known only from Arctic localities — Greenland, 

 Novaya Zemlya, and Finland. Its long tapering spring- 

 tip is very characteristic (Fig. 4, b). The other species. 

 X. maritimo, TuUb., has occurred on the shores of Fin- 

 land, Sweden, and Ireland. It furnishes an interesting 

 stage in the degeneration of the spring, since the spring- 

 tip is not clearly separated fiom the tooth which bears 

 it, the two being fused into a single pointed process 

 (Fig. 4. c). Though often found among rotting wrack, 

 Xenylla is, despite its name, far from exclusively mari- 

 time, since it inhabits the kingdom of Wurtemburg. and 

 has been found in northern Germany beneath the bark 

 of trees. 



Very similar in aspect to Xenylla is another blue black 

 insect, Anurida maritima (Guer.) (Fig. 5), when seen 

 with the unaided eye walking over rocks or tangled 



masses of seaweed. But Anurida is one of those Podurid 

 genera in which the spring is wanting ; a glance with a 



a 



O 0«§& 



Flo. 5. — Anurida maritima, iimgnilicd 20 times, a, ejes and 

 post-antemial organ of right side, hinlilv miignilied. 



lens enables one. therefore, readily to distinguish this 

 common insect from its scarce relation Xenylla. In its 

 perfect adaptation to a saltwater home, Anurida 

 maritima is the most characteristic of all our marine 

 Spring-tails.** For it seems never to bo found except 

 between the tide-marks, and may often be observed in 

 large companies on the surface of rock-pools. The insects' 

 presence here, however, is, according to the observations 

 of Prof. Moniez, involuntary. When the tide rises they 

 shelter in crevices of the rocks, among seaweed, or in 

 empty shells — wherever they can undergo immersion 

 without getting washed away. The last shock 

 of the waves as the tide recedes, often carries 

 them out of such shelters on to the surface of 

 the pools, where they collect together in largo 

 masses, climbing over one another. By the force of 

 the wind, the living mass may be carried to the edge 

 of the pool ; then the outside individuals at once do 

 their best to clamber up the rocks, never seeking to 

 regain the sea, though sometimes an insect may, despite 

 its efforts, fall once again into the water. The skin of 

 Anurida is covered with numerous very minute tuber- 

 cles ; these ensure that the surface film shall not be 

 broken, and thus prevent the insect from getting wetted. 

 Even a submerged individual is surrounded by a layer of 

 air. a gleaming silvery bubble being visible through the 

 water. 



As if to compensate them for the loss of the spring, 

 these lowly Podurids have a mysterious sense-organ on 

 either side of the head, between the feelers and the eyes, 

 which attains a high degree of complexity. In some of 

 the less degenerate Spring-tails — Isotoma, for example — 

 this organ consists of a simple elongate-oval depression, 

 but in the Podurids we find within the depression a 

 number of minute prominences, usually regularly 

 arranged and of a remarkably complex form. In Anurida 

 maritima, these prominences are seven in number, and 

 somewhat resemble the lights of a circular window 

 (Fig. 5, a). From the recent researches of M. Willemjtt 

 it seems that the structure of these prominences and of 

 the connected nerve-endings show that smell is the 

 function of the " post-antennal " organ. Some Podurids 

 (Xenylla, for example) have eyes but no post-antennal 

 organs ; others (as Lipura) post-antennal organs but no 

 eyes. Anurida rejoices in both. 



Probably Anurida maritima is common all round the 

 rocky parts of our coasts. It also inhabits the shores 

 of France and Heligoland, but it has not been traced into 



•• A. Laboulbene. " Recherehes siir I'Anurida maritimie." Ann. 

 Soc. Ent. France (4)„Vol. IV., 1864. 



++ V. Willcm. " Les Yeux et les Or(»ane8 post-antennaires des 

 Collcmboles." Ann. Soc. Entom. Belg., Vol. XLI.. 1807, pp. 22o-U. 



