opell (33 risultati)

Soft cover. Condizione: Very Good. Paginated iii, [1], 553-732, illustrated. Size: 11 x 8 7/16 inches. [otob: 60L].

Paperback. Condizione: Brand New. 172 pages. 9.00x6.00x0.39 inches. In Stock.

Paperback. Condizione: Brand New. 172 pages. 9.00x6.00x0.39 inches. In Stock.

Paperback. Condizione: Brand New. 104 pages. 8.25x8.25x0.24 inches. In Stock.

15pp, 4 figs, Printed Card Cover, VGC, All our offprints are original printed matter 'as issued' with bound card covers, they not modern reprints or photocopies.

New taxon: Tangaroa beattyi n. sp. 9 pp., 25 figs, gr. 8.

3 pp., 1 tab., gr. 8.

7 pp., 3 figs, 1 tab., gr. 8.

New taxa: Uloborus metae n. sp., Uloborus eberhardi n. sp., Philoponella subvittata n. sp. 10 pp. 38 figs, gr. 8.

New taxa: Lubinella morobensis n. gen., n. sp. 6 pp., 13 figs, gr. 8.

8 pp., 10 figs, gr. 8.

6 pp., 2 figs, gr. 8.

New taxon: Uloborus elongatus n. sp. 3 pp., 8 figs, gr. 8.

6 pp., gr. 85.

10 pp., 13 figs, 1 tab., gr. 8.

8 pp., 11 figs, 1 tab., gr. 8.

In uloborid spiders, eye loss is accompanied by increased visual angles, optical material investment, and potential visual acuity of the retained eyes. Relative to carapace volume, the six-eyed Hyptiotes cavatus and two four-eyed Miagrammopes species have greater retinal hemisphere areas and lens volumes than do the eight-eyed uloborids Waitkera waitkerensis, Uloborus glomosus, and Octonoba sinensis. In Waitkera, in which the eyes have little visual overlap, and in Miagrammopes, in which eye loss simplifies the spiders' patterns of visual overlap, increased retinal cell density enhances potential visual acuity. However, this occurs at the expense of potential retinal cell sensitivity. 8 pp., 2 figs, 3 tabs, gr. 8.

When visual fields of the primitive orb-weaver, Waitkera waitkerensis, are reconstructed using measurements taken from intact lenses and cross and longitudinal sections of the prosoma, they show that this species has complete visual surveillance, but that none of the visual fields of its eight eyes overlap. The more advanced orb-weaver, Uloborus glomosus, also has eight eyes, but each eye has a greater visual angle, giving this species a complex pattern of overlapping visual fields. Uloborids that spin reduced webs are characterized by reduction or loss of the four anterior eyes and other carapace modifications necessary for them to effectively monitor and manipulate their reduced webs. The eyes of these uloborids have greater visual angles than those of orb-weavers, resulting primarily from perimetric expansion of their retinal hemispheres. Additionally, the axes of their visual fields are more ventrally directed due to greater dorsal than ventral retinal expansion and to ventral redirection of the entire eye. Consequently, even though the anterior lateral eyes of the triangle-weaver Hyptiotes cavatus lack retinae, the species' six functional eyes permit complete visual surveillance and exhibit visual overlap. The single-line-weaver, Miagrammopes animotus, has lost its four anterior eyes, and with them much of the anterior vision and all of the visual overlap found in the other species. However, changes similar to those of H. cavatus permit this species to retain most if its dorsal and ventral visual surveillance. Thus, ocular changes act in consort to maintain relatively complete visual surveillance in the face of eye loss and other major carapace modifications necessary for the operation of reduced webs. 14 pp., 4 figs, 3 tabs, gr. 8.

Differences in web structure and cephalothorax features suggest that Hyptiotes cavatus should exert more force while monitoring its vertical triangle-web than Uloborus glomosus exerts while hanging beneath the hub of its horizontal orb-web. When this hypothesis was tested by measuring the force that instars of each species exerted on a horizontal thread, Hyptiotes cavatus was found to exert significantly more force throughout development than did Uloborus glomosus. This relationship holds when either first femur length or body weight is used as an index of spider size. 4 pp., 5 figs, 4.

Members of the genera Uloborus, Hyptiotes, and Miagrammopes have similar web-monitoring postures, but very different webs and tactics for monitoring them. Orb weavers of the genus Uloborus construct horizontal webs and hang from their hubs, whereas reduced-web uloborids construct vertical webs and monitor them from a single thread. To determine if changes in spider strength accompanied web reduction, resting and maximum force measurements were taken of a developmental series of Hyptiotes cavatus, Uloborus glomosus, Miagrammopes animotus, Miagrammopes pinopus, and an undescribed Costa Rican Miagrammopes using a glass needle strain gauge. Both carapace length and spider weight were used as indexes of spider size. Regression analyses of forces show that H. cavatus exerts the greatest relative force and Miagrammopes species the least. This is consistent with requirements for the operation of each web type: Hyptiotes cavatus tenses its entire triangular web and suddenly releases this tension when a prey strikes its web, whereas a Miagrammopes species jerks a single thread that has captured a prey. Within the genus Miagrammopes, the species with the most highly modified carapace expressed the greatest resting force. 7 pp., 12 figs, 3 tabs, 4.

In the Uloboridae, web reduction is accompanied by changes in opisthosomal shape, leg length, and web-monitoring tactics. These morphological changes make reduced-web spiders more cryptic and alter their leg leverage and centers of mass. When compared with the orb-weaver Uloborus glomosus, the irregular, reduced-web spider, Miagrammopes animotus, invests more mass in its prosoma and first legs. However, the latter species' elongate opisthosoma posteriorly shifts this region's center of mass, causing the relative position of its composite center of mass and the distribution of weight between its first and fourth legs to be similar to that of the orb-weaver. Like these species, the opisthosomal center of mass of the triangle-weaver, Hyptiotes cavatus, lies near its midpoint. However, the shorter first legs and rounder, heavier opisthosoma of Hyptiotes posteriorly shift its composite center of mass and distribute more of its weight onto its fourth legs. Consequently, the morphology of M. animotus can be adequately explained by its adaptiveness for web manipulation, balance, and weight distribution and the crypsis that these features confer as an ancillary advantage. In contrast, anatomical changes in H. cavatus are better explained as adaptations for web manipulation and crypsis. 9 pp., 7 figs, gr. 8.

10 pp., 2 figs, 1 tab., 8.

Observations of 22 orb-weaving species of the family Uloboridae show that these spiders assume one of four basic resting postures as they hang beneath the web's hub. The primitive pattern found in Tangaroa and Octonoba is characterized by all legs being spread and about equally flexed, whereas in Zosis the protracted first legs grasp the web at nearly the same point. Uloborus species typically assume a more cryptic posture characterized by acutely protracted and flexed first legs. Dense setal tufts proximal to the abruptly flexed leg segment provide outline camouflage and conceal the extended, shorter second legs which no longer support the body. In contrast, Philoponella species lack leg tufts and assume a compact posture with first legs folded against the sternum and only the last three pairs of legs grasping the web. The significance of these findings for uloborid classification and phylogeny is discussed. 8 pp., 8 figs, 1 tab., gr. 8.

In the family Uloboridae, web reduction is associated with changes in web monitoring posture and prosomal features. A spider must extend its first pair of legs directly forward to monitor the signal line of a reduced web. This posture is facilitated by shifts in prosomal musculature that cause reduced web uloborids to have a narrower anterior prosoma, a reduced or absent anterior eye row, and prominent posterior lateral eye tubercles. The eye tubercles and larger posterior eyes of these uloborids suggest that web reduction may also be accompanied by ocular changes that compensate for reduction of the anterior eyes by expanding the visual fields of the posterior eyes. A comparison of the visual fields of the eight-eyed, orb web species Octonoba octonaria and a four-eyed, reduced web Miagrammopes species was made to determine if this is true. Physical and optical measurements determined the visual angles of each species' eyes and the pattern of each species' visual surveillance. Despite loss of the anterior four eyes, the Miagrammopes species has a visual coverage similar to that of O. octonaria. This is due to (1) an increase in the visual field of each of the four remaining Miagrammopes eyes, accruing from an extension of the retina and an increase in the lens' rear radius of curvature, and (2) a ventral shift of each visual axis, associated with the development of an eye tubercle and an asymmetrical expansion of the retina. Miagrammopes monitor their simple webs from twigs or moss where they are vulnerable to predation. Therefore, maintenance of visual cover may enable them to detect predators in time to assume or maintain their characteristic, cryptic posture. It may also allow them to observe approaching prey and permit them to adjust web tension or prepare to jerk their webs when prey strikes. 6 pp., 3 figs, 4.

Uloborid cribellar silk consists of torus-shaped puffs. In Miagrammopes animotus the width of these puffs is about 36% that of the cribellum of the spider and shows a 2.3-fold increase in surface area during development. The cribellar spigot number increase 5.7-fold during development, although, relative to spider mass, it decreases by 34%. Cribellum width is the best predictor of both cribellar silk puff width and length and is as good a predictor of puff surface area as is cribellum surface area. Relative to cribellum width, the length of the calamistrum comb responsible for drawing fibrils from the cribellum changes little during development. The attachment points of cribellar silk to a parallel frame thread become more widely spaced during development, although the number of puffs they delimit changes little. 5 pp., 9 figs, 1 tab. 4.

The new Costa Rican species Philoponella herediae and its web are described and illustrated. The web consists of a horizontal orb-web beneath which converging, vertical threads are spun. New taxon: Philoponella herediae n. sp. 5 pp., 8 figs, gr. 8.

Uloborids that spin reduced webs more actively monitor them than those that construct orb webs. Hyptiotes use both their first and fourth legs to tense their triangle-webs, whereas Miagrammopes rely principally on their first legs to monitor and jerk the threads of their irregular webs. The respiratory systems of these spiders include tracheae that extend into the prosoma, bifurcate, and enter the legs. To determine if the legs responsible for active web-monitoring tactics have more extensive tracheal supplies, the total cross sectional area has been computed of the tracheae entering the legs of mature female orb web and reduced web uloborids. Each leg's value has been divided by the cross sectional area of the tracheal trunks that enter the prosoma. These indexes reveal no significant differences between the relative tracheal supplies of the orb weavers investigated (Waitkera waitkerensis, Tangaroa beattyi, Uloborus glomosus). But the first, third, and fourth legs of H. cavatus and the first legs of M. animotus and M. pinopus have greater relative tracheal supplies than those of the three orb weaving species. Relative to leg volume, the first and fourth legs of H. cavatus have the greatest and the first legs of Miagrammopes species the next greatest tracheal supplies. When tracheal lengths are considered, these differences in potential oxygen supplies remain, showing that area differences do not simply compensate for differences in the distances over which oxygen must diffuse. These differences are leg-specific and not species-specific, and uloborids with the most extensive tracheal supplies are found in moist habitats. Thus the observed differences are best explained as adaptations to meet the greater oxygen demands of legs responsible for active web-monitoring tactics and not as adaptations to reduce respiratory water loss. 5 pp., 4 figs, 3 tabs, 4.

5 pp., 2 figs, gr. 8.

14 pp., 5 figs, 5 tabs, gr. 8.

New taxa: Conifaber n. gen., Conifaber parvus n. sp., Uloborus conus n. sp., Uloborus albolineatus n. sp., Uloborus bispiralis n. sp. 35 pp., 48 figs, 8.