gerardiae (Lacaze-Duthiers, 1865), sends such rootlets into its host as
would justify the term Rhizothoracida. The small sinuous
segmented body is enclosed, except for one small opening, in an
enormous sac-like carapace, between the lamellae of which are
protruded from the body the ovary and “liver,” both large,
bifurcate and ramified. It is this sac-like and not valvular
carapace, therefore, that justifies the term Ascothoracida. But Synagoga
mira, Norman, 1888 (Brit. Assoc. Report for 1887), has the body
covered by two nearly circular valves instead of a sac. Petrarca
bathyactidis, G. H. Fowler (Quart. Journ. Mic. Sci., 1889, vol.
xxx. pt. ii. p. 115), has a bilobed carapace, ventrally open;
Dendrogaster astericola, Knipovitch (Biologisches Centralblatt, 1891,
x. 707), is a multilobular sac, with apparently indistinct
segmentation of the body proper on the dorsal side. For this
highly problematic group the original authorities should by all
means be consulted. The student may then be asked to compare
the account of Synagoga mira both with the figure of the cypris-stage
of Dendrogaster astericola and with the figure of the “indeterminate
animal found on Gerardia,” about which Lacaze-Duthiers
asks, “Is it the cypris-stage of Laura?” (Mém. Acad. Sci.,
1883, xlii. 160, pl. 1, fig. 102). S. mira was found, like Laura
gerardiae (fig. 3), in the Mediterranean, and found like it attached
to an antipatharian. Its six pairs of limbs are not like the bare
and simple feet of the Laura, but two-branched and setose as in the
ordinary cypris-stage of the cirripede. The conclusion, therefore,
from these facts and from the suggested comparisons, seems at least
extremely probable that the question asked by H. Lacaze-Duthiers
should be answered in the affirmative, and that S. mira is either
the cypris-stage of Laura gerardiae or of some congeneric species.
In Lacaze-Duthiers's highly-elaborated memoir it should be noticed
that he uses the term “cirrhes” rather misleadingly, not for
cirrhiform feet, but as the equivalent of setae. Also he gives two
different reckonings of the segmentation, counting first eleven
body segments without the caudal furca (p. 40), and then the
caudal furca as itself the eleventh segment (p. 41). Of Petrarca
the development is not yet known. The points of agreement and
difference between it and Laura are carefully drawn out in the
essay by Dr G. H. Fowler, who inclines to favour a close relationship
between the Thyrostraca and Ostracoda. To the extreme
development of the carapace in Laura, as compared with the
segmented body, it would be difficult to find among crustaceans
any analogy more striking than that of the great ovarial expansions
in Nicothoe astaci, the little copepod parasite of the common lobster.
The compactness of the class Crustacea is generally admitted; of
the precise affinities of its subdivisions there is still much to learn.
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Fig. 3.—A, Laura gerardiae; B, Carapace slit open to show the body proper. |
THYRSUS (the latinized form of Gr. θύρσος, a stem or stalk) the wand or staff of Dionysus (Bacchus), the Bacchants and Maenads and the votaries taking part in his orgiastic rites. As commonly represented on the monuments it was a straight staff terminating in a pine cone, a ribbon or fillet being attached to the head below it. Another form terminated in a bunch of grapes and vine leaves, or ivy-berries and leaves. The pine-cone or bunch of leaves was sometimes supposed to cover the head of a spear, and the thyrsus was termed θυρσόλογχος (see Dionysus and Mystery).
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(The illustrations in this article are all after H. Uzel, Monographie der Ordnung Thysanoptera, and are used with his permission.) |
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Fig. 1.—Limothrips denticornis, female, Europe. |
THYSANOPTERA (θύσανος, a fringe, and πτερόν, a wing), a term used in zoological classification for a small order of the class Hexapoda (q.v.). The minute insects included in it, which haunt blossoms and leaves, are fairly well known to gardeners by the name Thrips, a generic term used by Linnaeus for the four species of the group which he had examined and relegated to the order Hemiptera. The term Thysanoptera was first used by the Irish entomologist A. H. Haliday (1836), who made a careful study of the British species and recognized that their structural peculiarities required ordinal separation. H. Burmeister in 1838 also considered that these insects should form a distinct order, for which he proposed the name Physopoda, with reference to the bladder-like outgrowths (fig. 3) on the feet. Since then various authors have incorporated the Thysanoptera with one of the large orders, some, following Linnaeus, regarding them as Hemiptera, others grouping with them the Orthoptera or “Pseudoneuroptera.” But all recent students agree with Haliday and Burmeister in allowing the Thysanoptera to rank as a distinct order, showing affinities on the one hand with the Corrodentia (book-lice and biting lice) and on the other with the Hemiptera (cicads, bugs, &c.).
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| (After H. Uzel.) | |
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Fig. 2.—Head of Aeolothrips fasciata, face view, showing eyes, bases of feelers and jaws. | |
| a, | Clypeus. |
| b, | Membrane between clypeus and labrum. |
| c, | Labrum. |
| d, | Mandible. |
| e, | Unpaired piercer (? inner lobe of left maxilla). |
| f, | Maxilla. |
| g, | Its palp. |
| h, | Second maxillae, forming labium. |
| i, | Labial palp. |
Characters.—The Thysanoptera, small insects with firmly chitinized cuticle, are recognized by the combination of imperfectly suctorial jaws—the mandibles acting as piercers and maxillae retaining their palps (see fig. 2)—with the presence of two pairs of excessively narrow wings (fig. 1), which are partly or completely surrounded by elongate delicate bristles forming a fringe. Other important structural features are mentioned below. In their life-history the Thysanoptera belong to the Exopterygote division of the Hexapoda (q.v.). The newly hatched insect closely resembles the parent, and the wing-rudiments appear externally on the second and third thoracic segments; but before the final moult the nymph remains quiescent, taking no food. Its condition thus recalls the pupal instar of the higher (Endopterygote) Hexapoda; and the Thysanoptera, though few in number, are seen to be of great interest to the student, exhibiting at once transition between the biting and the suctorial mouth, and the passage from “incomplete” to “complete” metamorphosis.
Structure.—The head is usually quadrangular in form with small but prominent compound eyes (fig. 2), whose facets are relatively large and convex; three ocelli may also be present on the vertex. The feelers are inserted close together (fig. 2) on the extreme front of the head; they exceed the head in length, but they are composed of only from six to nine segments, which are beset with prominent spines, some of the latter appearing to be organs of special sense. The mouth, with its jaws, forms a conical outgrowth which projects backwards, so that its apex lies beneath the prothorax. The labrum (fig. 2 c), which encloses the cone in front, is irregularly triangular in shape. Behind the mouth the two maxillae of the second pair are intimately
