Fig. 156.—Alar organs and one side of thorax of Phyllium crurifolium: A, tegmen; B, rudiment of wing; C, pronotum; D, anterior division of mesonotum; E, posterior division; F, metanotum; a, b, c, d, e, chief wing-nervures; a, mediastinal; b, radial; c, ulnar; d, dividens?; e, plicata?.
In the female Phyllium the hind wings are not present, being represented by a minute process (Fig. 156, B). The tegmen of the female Phyllium is, from various points of view, a remarkable and exceptional structure. It is the rule that when there is in Insects a difference between the alar organs of the two sexes it is the male that has them largest; this is the case in Phyllium so far as the hind wings are concerned, but in the fore-wings the rule is departed from, the leaf-like tegmina of the female being very much larger than the rudimentary wing-covers of the male. In Phasmidae it is the rule that the tegmina are atrophied, even when the hind wings are largely developed. This is the case in the male of Phyllium, but in the female this normal condition is reversed. Although the alar organs of Phasmidae have received hitherto but a small amount of attention, it is probable that the female tegmen of Phyllium is as peculiar morphologically as it is in other respects. In Fig. 156 we give an accurate representation of the chief nervures in the tegmen of a female P. crurifolium. It is interesting to compare this with the diagrams we give of the tegmina of a Blattid (Fig. 121) and of an Acridiid (Fig. 167); the tegmen of the Phyllium is very different, the radial vein and all the parts behind it being placed quite close to the posterior edge of the structure. A similar view is taken by both Redtenbacher and Brauer. The latter says,[[198]] "In Phyllium (the walking-leaf) almost the whole of the front wing is formed by the praecostal and subcostal fields; all the other fields with their nervures, including even the costa, are compressed towards the hind margin into a slender stripe. In the hind wing the costa is, however, marginal." Unfortunately no examination appears to have been made of the male tegmen, so that we do not know whether that of the female differs from it morphologically as strongly as it does anatomically. It is, however, clear that the tegmina of the female Phyllium not only violate a rule that is almost universal in the Insecta, but also depart widely from the same parts of its mate, and are totally different—and, for a Phasmid, in an almost if not quite unique fashion—from the other pair of alar organs of its own body.
Fig. 157.—Egg of Phyllium scythe. (After Murray.) A, The whole egg, natural size; A', magnified; B, the capsule broken, showing the true egg inside, natural size; B', magnified.
Fig. 158.—Portion of a longitudinal section of the egg capsule of Phyllium crurifolium: a, external; b, middle; c, inner zones; d, elongate alveoli. × 100. (After Henneguy.)
We have already alluded to the resemblance to seeds displayed by the eggs of Phasmidae. The eggs of Phyllium have been studied by several entomologists, and their resemblance to seeds excites general astonishment. Murray describes the egg-capsule of Phyllium scythe, and says: "It looks uncommonly like some seeds; if the edges of the seed of Mirabilis jalapa were rubbed off, the seed might be mistaken for the egg. The ribs are all placed at equal distances, except two, which are wider apart, and the space between them flatter, so that on the egg falling it rolls over till it comes to this flatter side, and there lies.... At the top there is a little conical lid, fitting very tightly to the mouth.... On removing the lid we see a beautiful porcelain chamber of a pale French-white colour, bearing a close resemblance to the texture of a hen's egg, but it is not calcareous, and has more the appearance of enamel." The eggs of P. crurifolium have been examined by Joly and Henneguy; their account confirms that of Murray. Henneguy adds that a prominent lozenge on the egg represents the surface by which the achene of an umbelliferous plant is united to the column, and that the micropyles are placed on this lozenge. The minute structure of the capsule has also been examined by several entomologists; and Henneguy,[[199]] who has described and figured some of the details of the capsule of P. crurifolium, says, "Almost every botanist, on examining for the first time a section of this capsule, would declare that he is looking at a vegetable preparation."
We may remark that, although there is difference of opinion on the point, the evidence extant goes to show that the egg-capsules are formed in the egg-tubes, only one egg being produced at a time in a tube,[[200]] the others in it remaining quite rudimentary.
About 600 species of the family are known; there are only four or five kinds found in Europe, and they are all confined to the south, only one of them extending as far north as Central France. The males of these European Bacilli are extremely rare in comparison with the females, which are common Insects. Phasmidae are of almost universal distribution in the warm parts of the world, and even the species whose individuals are of large size seem to be able to continue their existence in comparatively small islands. Australia is perhaps the region where they are most largely developed at present. Macleay says of Podacanthus wilkinsoni that it is rare in any part of Australia to find in the summer season a gum-tree without a few of these Insects grazing on it; and occasionally this Insect has been so abundant there that the trees for miles around have been denuded of their foliage by it, and the dead and dying Insects have been found lying beneath the trees almost in heaps. There are several Phasmidae in New Zealand, all wingless forms, and different from those found in Australia. In Brazil a species of the genus Prisopus has the peculiar habit of seeking shelter under the stones submerged in the mountain streams; to enable it to do this it is remarkably constructed, the under side of the body being hollowed, and various parts set with a dense fringe of hairs; the Insect is supposed to expel the air from the body in order to adhere to the upper surface of a stone, where it sits with its fore legs extended in front of its head, which is directed against the current. Attention has been called to a still more remarkable form said to be allied to the Prisopi, by Wood-Mason,[[201]] who calls the Insect Cotylosoma dipneusticum. This Insect is apparently known only by a single example of the female sex; it is 3 or 4 inches in length, has rudimentary organs of flight, and along the lower margins of the metathorax there are said to be on each side five conspicuous fringed plates of the nature of tracheal gills; these coexist with open stigmata for aerial respiration, as in the imago of Pteronarcys. The writer has examined this curious Insect, and thinks it very doubtful whether the plates are branchiae at all. The locality for this Insect is the island of Taviuni, not Borneo, as stated by Wood-Mason. These and one or two Acridiidae are the only Insects of the Order Orthoptera at present believed to possess aquatic habits.
