Page:EB1911 - Volume 09.djvu/688

Rh traced details of the growth of embryo and of pupa among the Diptera, and A. Kovalevsky in 1871 first described the formation of the germinal layers in insects. Most of the recent work on the embryology of insects has been done in Germany or the United States, and among numerous students V. Graber, K. Heider, W. M. Wheeler and R. Heymons may be especially mentioned.

The work of de Réaumur and de Geer on the bionomics and life-history of insects has been continued by numerous observers, among whom may be especially mentioned in France J. H. Fabre and C. Janet, in England W. Kirby and W. Spence, J. Lubbock (Lord Avebury) and L. C. Miall, and in the United States C. V. Riley. The last-named may be considered the founder of the strong company of entomological workers now labouring in America. Though Riley was especially interested in the bearings of insect life on agriculture and industry— (q.v.)—he and his followers have laid the science generally under a deep obligation by their researches.

After the publication of C. Darwin’s Origin of Species (1859) a fresh impetus was given to entomology as to all branches of zoology, and it became generally recognized that insects form a group convenient and hopeful for the elucidation of certain problems of animal evolution. The writings of Darwin himself and of A. R. Wallace (both at one time active entomological collectors) contain much evidence drawn from insects in favour of descent with modification. The phylogeny of insects has since been discussed by F. Brauer, A. S. Packard and many others; mimicry and allied problems by H. W. Bates, F. Müller, E. B. Poulton and M. C. Piepers; the bearing of insect habits on theories of selection and use-inheritance by A. Weismann, G. W. and E. Peckham, G. H. T. Eimer and Herbert Spencer; variation by W. Bateson and M. Standfuss.

—References to the works of the above authors, and to many others, will be found under and the special articles on various insect orders. Valuable summaries of the labours of Malpighi, Swammerdam and other early entomologists are given in L. C. Miall and A. Denny’s Cockroach (London, 1886), and L. Henneguy’s Les Insectes (Paris, 1904).

ENTOMOSTRACA. This zoological term, as now restricted, includes the Branchiopoda, Ostracoda and Copepoda. The Ostracoda have the body enclosed in a bivalve shell-covering, and normally unsegmented. The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the postoral appendages normally branchial. The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial.

Under the heading the Entomostraca have already been distinguished not only from the Thyrostraca or Cirripedes, but also from the Malacostraca, and an intermediate group of which the true position is still disputed. The choice is open to maintain the last as an independent subclass, and to follow Claus in calling it the Leptostraca, or to introduce it among the Malacostraca as the Nebaliacea, or with Packard and Sars to make it an entomostracan subdivision under the title Phyllocarida. At present it comprises the single family Nebaliidae. The bivalved carapace has a jointed rostrum, and covers only the front part of the body, to which it is only attached quite in front, the valve-like sides being under control of an adductor muscle. The eyes are stalked and movable. The first antennae have a lamellar appendage at the end of the peduncle, a decidedly non-entomostracan feature. The second antennae, mandibles and two pairs of maxillae may also be claimed as of malacostracan type. To these succeed eight pairs of foliaceous branchial appendages on the front division of the body, followed on the hind division by four pairs of powerful bifurcate swimming feet and two rudimentary pairs, the number, though not the nature, of these appendages being malacostracan. On the other hand, the two limbless segments that precede the caudal furca are decidedly non-malacostracan. The family was long limited to the single genus Nebalia (Leach), and the single species N. bipes (O. Fabricius). Recently Sars has added a Norwegian species, N. typhlops, not blind but weak-eyed. There are also now two more genera, Paranebalia (Claus, 1880), in which the branchial feet are much longer than in Nebalia, and Nebaliopsis (Sars, 1887), in which they are much shorter. All the species are marine.

—In this order, exclusion of the Phyllocarida will leave three suborders of very unequal extent, the Phyllopoda, Cladocera, Branchiura. The constituents of the last have often been classed as Copepoda, and among the Branchiopods must be regarded as aberrant, since the “branchial tail” implied in the name has no feet, and the actual feet are by no means obviously branchial.

Phyllopoda.—This “leaf-footed” suborder has the appendages which follow the second maxillae variable in number, but all foliaceous and branchial. The development begins with a free nauplius stage. In the outward appearance of the adults there is great want of uniformity, one set having their limbs sheltered by no carapace, another having a broad shield over most of them, and a third having a bivalved shell-cover within which the whole body can be enclosed. In accord with these differences the sections may be named Gymnophylla, Notophylla, Conchophylla. The equivalent terms applied by Sars are Anostraca, Notostraca, Conchostraca, involving a termination already appropriated to higher divisions of the Crustacean class, for which it ought to be reserved.

1. Gymnophylla.—These singular crustaceans have long soft flexible bodies, the eyes stalked and movable, the first antennae small and filiform, the second lamellar in the female, in the male prehensile; this last character gives rise to some very fanciful developments. There are three families, two of which form companies rather severely limited. Thus the Polyartemiidae, which compensate themselves for their stumpy little tails by having nineteen instead of the normal eleven pairs of branchial feet, consist exclusively of Polyartemia forcipata (Fischer, 1851). This species from the high north of Europe and Asia carries green eggs, and above them a bright pattern in ultramarine (Sars, 1896, 1897). The Thamnocephalidae have likewise but a single species, Thamnocephalus platyurus (Packard, 1877), which justifies its title “bushy-head of the broad tail” by a singularity at each end. Forward from the head extends a long ramified appendage described as the “frontal shrub,” backward from the fourth abdominal segment of the male spreads a fin-like expansion which is unique. In the ravines of Kansas, pools supplied by torrential rains give birth to these and many other phyllopods, and in turn “millions of them perish by the drying up of the pools in July” (Packard). The remaining family, the Branchipodidae, includes eight genera. In the long familiar Branchipus, Chirocephalus and Streptocephalus the males have frontal appendages, but these are wanting in the “brine-shrimp” Artemia, and the same want helps to distinguish Branchinecta (Verrill, 1869) from the old genus Branchipus. Of Branchiopsyllus (Sars, 1897) the male is not yet known, but in his genera of the same date, the Siberian Artemiopsis and the South African Branchipodopsis (1898), there is no such appendage. Of the last genus the type species B. hodgsoni belongs to Cape Colony, but the specimens described were born and bred and observed in Norway. For the study of fresh-water Entomostraca large possibilities are now opened to the naturalist. A parcel of dried mud, coming for example from Palestine or Queensland, and after an indefinite interval of time put into water in England or elsewhere, may yield him living forms, both new and old, in the most agreeable variety. Some caution should be used against confounding accidentally introduced indigenous species with those reared from the imported eggs. Those, too, who send or bring the foreign soil should exercise a little thought in the choice of it, since dry earth that has never had any Entomostraca near it at home will not become fertile in them by the mere fact of exportation.

2. Notophylla.—In this division the body is partly covered by a broad shield, united in front with the head; the eyes are sessile, the first antennae are small, the second rudimentary or wanting; of the numerous feet, sometimes sixty-three pairs, exceeding the number of segments to which they are attached, the first pair are more or less unlike the rest, and in the female the eleventh have the epipod and exopod (flabellum and sub-apical lobe of Lankester) modified to form an ovisac. Development begins with a nauplius stage. Males are very rare. The single family Apodidae contains only two genera, Apus and its very near neighbour Lepidurus. Apus australiensis (Spencer and Hall, 1896) may rank as the largest of the Entomostraca, reaching in the male, from front of shield to end of telson, a length of 70 mm., in the female of 64 mm. In a few days, or at most a fortnight, after a rainfall numberless specimens of these sizes were found swimming about, “and as not a single one was to be found in the water-pools prior to the rain, these must have been developed from the egg.” Similarly, in Northern India Apus himalayanus was “collected from a stagnant pool in a jungle four days after a shower of rain had fallen,” following a drought of four months (Packard).