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GRAPHIC MICROSCOPY.

By E. T. DRAPER.

No. XXIII.-POLY SIPHONIA FASTIGIATA.

N most of the larger littoral algæ, but rarely beyond lowwater mark, may be found the delicate and elegant parasitical Polysiphonia fastigiata; it grows abundantly on the fronds of Fucus nodosus, and often entirely covers the long thong-like

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of Fucus vesiculosus. Although common, and apparently uninviting as an object of beauty, the microscope reveals its extreme elegance. It is discovered forming dense globular tufts two or three inches long, about the thickness of a horse hair at the base, expanding in parallel branches pointing upwards, somewhat rigid. Microscopically, the frond is found to be filiform, and articulated, repeatedly forked, marked externally with striæ, interrupted at the joints, and generally the structure is disposed in a series round a central cylindrical internal cavity; the colour brownpink-purple-black when dry. Although the mode of propagation has until recently been somewhat obscure, it is ascertained that there are two kinds of fructification, in distinct plants. Tetraspores which at maturity divide into parts, generally four, or more; and antheridia, oblong bodies, rounded at the extremities, produced in fascicles on the summits of the ramuli, and subdivided into parts. The drawing shows the apices of a frond with these reproductive organs. They are so extremely abundant in the early spring months, as to give a very conspicuous yellow colour to the tufts on which they are produced. Minute marine algæ, for microscopical observation No. 251.-NOVEMBER 1885.

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and preparation may be cultivated, or rather kept in a growing condition for a few weeks in small vases of sea-water, in a cool and shady position, under an equable temperature. It is of importance that the plants should be attached to a portion of the substance on which they are found growing; for permanent preservation, the lace-like fragment should be floated on fresh water, lifted carefully on the usual glass slip, well drained and immersed in glycerine jelly under a thin cover; no pressure should be used. Dry specimens are mounted, but they rarely exhibit the integrity or delicate features of the fructification.

Crouch End.

ON MIMICRY IN DIPTERA.

WHE

HEN will some of our entomologists who have collected and studied nearly every Lepidopterous insect known to inhabit Great Britain, devote even a small amount of attention to the other orders of insects? The Coleoptera certainly have received a fair amount of attention, but the Diptera have been sadly neglected.

And this unpopularity is scarcely deserved by them, for albeit they do not possess the brilliant colouring and large dimensions of some butterflies, many of them are extremely elegant; while their points of interest will be found to be almost more varied and more striking than those of the Lepidoptera.

Any who would take the trouble to catch and pin out the Diptera found in their neighbourhood might possibly discover new habitats, if not new species; in any case, such a collection would greatly assist our knowledge of the distribution of the various species.

Those who travel miles to find a rare moth are usually only going over old ground that has been visited by scores of collectors bent on the same errand, and instead of increasing our knowledge of the insects, are only lending their small aid to the extermination of the species.

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The subject of this paper, is, however, one which opens up a wide field for observation, and in which much valuable information might be obtained even by those who have not sufficient leisure to form a local collection.

Many of the species mentioned below are among our commonest insects, while other not rare flies may prove on observation to be cases of mimicry.

One of the first things that a beginner at "fly catching" would notice is the extraordinary similarity between some of these insects and the bees and wasps. In showing my Diptera to friends, I notice that they constantly remark, "That is a bee, surely?" or "That is a wasp?" And the editors of "Little Folks" fell into the same error some years since, for I have before me a volume in which, among other instructive paragraphs for the young, I find one on "Busy Bees," accompanied by a very fair woodcut of several flies including Stratiomys, Tabanus, and even Tipula, but without a single bee or wasp among them.

That the Romans and other ancients evidently made a very similar mistake, owing to this resemblance, we shall see presently.

Beginning with the Stratiomyidæ, we find among the species of Stratiomys a considerable similarity to bees, especially when flying.

My specimen of S. furcata was captured under the impression that it was probably a bee, especially as the insect, when settling on a plant, folded its wings over the back in the same manner as do the bees.

Asilus crabroniformis (SCIENCE-GOSSIP, 1876, p. 156, Fig. 85) is so called from its having rather the habit of a hornet when on the wing, but when captured, it is seen to be so entirely unlike a hornet that, without further evidence on the subject, I can scarcely believe it to be a case of mimicry. Much more is Laphria ephippium, another insect of this family, like a bee. This fly occurs in many places on the Continent.

In the Leptidæ, I observed a remarkable case of undoubted mimicry, the mimicked insect being in this case, not a Hymenopteron, but a Neuropteron. I was walking along a lane in Warwickshire one June, some two or three years back; the scorpion fly (Panorpa communis), a neuropterous insect, familiar to all who live in the country, was extremely abundant, the hedges swarming with them, and after netting one or two of these, I thought I had captured another, but on examining it, I found it had two wings instead of four, and was easily recognised as Leptis scolopacea. I have placed the two insects in my collection side by side, and even when compared closely they possess considerable similarity with the wings folded.

In both, the wings are mottled with brown spots, the legs are longish and rather thin, and the abdomen is also slender.

But when settling on a hawthorn bush, the insects

were only with difficulty to be distinguished from one another, so that it is beyond doubt that, the scorpion fly, of which the body is rather hard, not being a very palatable meal for birds, the Leptis takes advantage of its similarity with this insect to escape being eaten, it being a softer bodied insect and therefore better food for birds. The fact that the Panorpa was by far the more abundant insect of the two is in corroboration of this.

The species of Bombylius, although called humblebee flies by some entomologists, do not much resemble any of our British species of Bombus. They feed on the juice of flowers, as does the humming-bird moth.

Though rather like some Apidæ, my observations would lead me to give my opinion against their being cases of mimicry, but perhaps that may be because I have not found the mimicked insect. As they dart about quickly, they may not need protection.

But it is among the flies of the family Syrphidæ that we find the most singular resemblance with Hymenoptera. Who has not seen the ubiquitous drone fly (Eristalis tenax) buzzing on the window pane, or, in late autumn, crawling wearily along the sill, and who has not mistaken it for a bee (Apis mellifica)? I have but to go into the garden and watch a patch of flowers; there, beside the numerous bees which come to gather honey, I am sure to find some of these flies. And I have to look twice before pronouncing them to be flies. If I take one of them in my fingers, some non-entomological friend will certainly exclaim, "Take care it does not sting you!"

Baron C. R. Osten Sacken has pointed out that the belief, universal among the ancients, that bees originated from carcases of dead animals (oxen, &c.), undoubtedly owes its origin to this resemblance. That belief is often mentioned in their writings (for instance it is alluded to at great length in Virgil's "Georgics," book iv. verses 285 et seq.), and has been reproduced by the earlier modern writers, such as Aldrovandi ("De Anim. Insectis," p. 58, edit. 1602), and Moufet (Theatr. Insect., p. 12).

The rat-tailed larvæ of Eristalis thrive in putrefying animal matter, and the very natural explanation of the superstition is that the perfect insects were mistaken for bees.

Eristalis æneus, as well as Cheilosia chrysocomus closely resemble some of the Andrenidæ, both in colouring and in general appearance.

E. floreus, on the other hand, takes after the wasps in its colouring; some specimens of this fly would be mistaken for wasps by any but an entomologist.

In that respect it is not singular, for several Syrphide are somewhat wasp-like when flying, but perhaps the best imitation of a wasp is that afforded by Chrysotoxum arcuatum and C. octomaculatum. The latter species is rare in England, but at Heidelberg some few summers past, when wasps (Vespa vulgaris)

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were very troublesome, there was rather an abundance of C. octomaculatum. The yellow and black markings on the abdomen, the wings with a brownish tinge, especially along the anterior margin, and even the long antennæ, all combine to produce an appearance very like a wasp. Owing to this fly being about when wasps are plentiful, it doubtless obtains some protection from this similarity.

Still more closely does Volucella bombylans mimic several species of humble-bee (Bombus), moreover it is subject to considerable variety, and each of the different forms exactly reproduces the colouring of a corresponding type of Bombus.

In Britain we have two varieties, distinguished by many entomologists as different species, V. bombylans and V. plumata.

Now in the former the thorax and abdomen are black, the thorax is covered with black hairs, while the tip of the abdomen is clad with hairs of an orange-brown colour. This is precisely the colouring of Bombus lapidarius and B. rupestris. In the latter, most of the thorax is clothed with yellowish hairs. There are patches of yellow on the sides of the abdomen near its base, while its tip is covered with whitish hairs-an arrangement of colour almost identically the same as in Bombus lucorum, B. collinus, B. pratorum and some others.

So much for the British forms of the fly, but it does not confine itself to imitating the colouring of only two kinds of Bombus. Baron Osten Sacken has kindly sent me a copy of a short review by him * of a Russian work by J. Portchinsky on the Diptera of the Caucasus resembling Bombus. M. Portchinsky finds that in the Caucasian mountains the humblebees (Bombus eriophorus, niveatus, Caucasicus) are all characterised by the prevalence of white hairs on various parts of the body. The plain black and orange coloured humble bees, like B. lapidarius, are entirely absent. It is therefore remarkable that in this region the black and orange variety of Volucella bombylans is absent, as though it had no cause for existing, while in its place a variety (V. bombylans, var. Caucasica) is found, which is unknown elsewhere in Europe, and in which the thorax and the base of the abdomen are clad with white hairs, after the manner of the humble bees of the Caucasus. A translation of M. Portchinsky's work is much to be desired.

All entomologists are aware of the great resemblance which obtains between the hornet clearwing moth (Trochilium apiforme) and the hornet (Vespa crabro). Not less striking is the resemblance to the latter insect of a fly (Milesia crabroniformis) which, though not found in England, is abundant in many parts of France and Italy. It is exactly the size of an average hornet, the colouring of the thorax abdomen and legs is very nearly the same, while even the wings are of a brownish tinge, similar to that of the hornet's wing.

* "Wiener Entomologische Zeitung," i. (1882), Heft 9.

When first I saw one of those flies buzzing round a trellis at Mentone some seven years ago, I captured it under the impression that it was a hornet.

At Cadenabbia, on the Lake of Como, Milesia crabronifornis was a common insect, and although it was doubtless protected from the attacks of birds by its likeness to a hornet, it sometimes suffered for its resemblance, for I have seen the natives try to kill it, and no amount of explanation could shake their firm conviction that it was a hornet.

Thus in one family of Diptera we have flies mimicking several types of common Hymenoptera; the bee, the wasp, the andrena, two forms of humble bee, and the hornet. The similarity is so great in these instances, particularly when the insects are alive and in motion, that no doubt can exist that they are cases of protective mimicry.

There are, however, other instances of resemblance between Diptera and Hymenoptera on which it is not so easy to decide whether they be cases of mimicry or not.

Before concluding, I should like to mention two that have come before my notice. Comparing a specimen of Myopa ferruginea with a species of Nomada (probably N. lateralis), I was at once struck by their general similarity, and remarked that even the whitish patches on the abdomen of Nomada were represented by light spots on the body of Myopa. The likeness between these two insects has previously been observed by entomologists. The other case which occurred to me only just lately, is the resemblance between Mesembrina meridiana and some of the Anthophore, as A. retusa.

The fly, one of the Muscidæ, has rather curious colouring, it is entirely black, with the exception of the wings, which, though pale grey towards the tips, are of a brilliant orange-yellow near the base. The Anthophoræ are black, but collect a quantity of pollen on their hind legs. They are about the size of M. meridiana. Now is it not highly probable that the yellow at the base of the wings of M. meridiana reproduces, when the insect is flying, the appearance of the pollen on the tibiæ of Anthophora ?

Whether the last two are cases of protective mimicry, could only be ascertained by open-air observation. If the flies are frequently seen associated with the insects they copy, or are found in similar places and seasons, we may fairly suppose them to be so. With regard to the Myopa, I fancy I have caught it and Nomada near the same spot, but as I knew scarcely any entomology at the time, and was quite a young boy, I may easily be mistaken. Were Diptera studied a little more, we should doubtless find numerous other cases of mimicry among them, including some that are now quite unknown. Let us therefore earnestly hope that some entomologists will employ their leisure in further investigations on this most interesting subject. G. H. BRYAN. Peterhouse, Cambridge.

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