W. T. M. Forbes.
The Second Abdominal Pleurite in the Higher Coleoptera.
Psyche 32(6):290-292, 1925.

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290 Psyche [December
molds the developing organism so uniformly true to type, and of the condition which now and then may cause the development of an individual in some detail to halt a little short of, to over-run, or otherwise to deviate from its due course. It is truly an obligation, very generally neglected by Amer- ican entomologists however, to record each example of insect teratology simply and directly, just as it is observed; but the fulfilment of this obligation is by no means accompanied by any additional obligation to offer at the same time an explanation or interpretation of the phenomenon. Indeed, in the present state of our knowledge of such phenomena any attempt to interpret individual examples is rarely demanded, or even scientifically justifiable.
THE SECOND ABDOMINAL PLEURITE IN THE HIGHER COLEOPTERA
BY W. T. M. FORBES,
Cornell University, Ithaca, New York.
It is generally recognized that the first sternite of the ab- domen is membranous in all the holometabolous insect*^. As to the first pleurite and the second segment there takes place a gradual reduction, which reaches such a stage that in the ma- jority of Coleoptera the first two segments are represented by a more or less membranous dorsal and spiracular region only. The usual statement is that the first segment of the ab- domen has atrophied, and that the second is ventrally absent and laterally fused with the third. An examination of fresh specimens shows that this is not strictly the case. In fact the second pleurite may disappear in at least two different ways. In the Bastrychiformia (Fig. 1, A) the first stage seems to have been an infolding of the whole subspiracular region of the first two segments, so that the hind coxa comes to lie against the anterior edge of the third segment or even overlap it some- what. In this way the two segments are completely buried from view, and their sclerites become more or less completely de-



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19251 Second Abdominal Pleurite in Higher Coleoptera 291 chitinized, except on the mid-ventral line where the second segment takes the form of a strong keel, filling the space between the coxse. The second pleurite is frequently completely mem- branous, but in the species figured (Trypopitys) it still shows some feeble chitinization and even a few rudimentary hairs .~AX'
Fig. 1.
Each figure gives a ventro-lateral view of the junction of thorax and abdomen, with the articulation separated to expose the membranous struc- tures. The variously shaded areas represent sclerites, the plain white, mem- brane. Hairy surfaces indicated, pads for the reception of the elytra stippled, the plain chitin striated.
Ai Membranous first sternal region of abdomen. Aa Membranous second sternal region of abdomen. epsa Metepisternum (the hairy ventral portion only is exposed when the elytra are closed). epmg Metepimeron
(apex uncovered in Trypopitys, wholly covered in Hemicrepidius). pis, play
p14 Pleurites of successive segments of abdomen. pli rudimentary chitinous nodule, perhaps representing the first pleurite. stn Metasternum of thorax. stA2, stA3, Successive sterna of abodmen. art. Articular facets in- iicating the junction of metepimeron and second (or third) pleurite. WP Wing process.
Heavy solid line.
Edge of superficial portion of pleura, adjoining pleura of thorax in life.
Heavy broken line. Edge of coxal cavity on surface of third sternite.
Fig. 1, A. Bostrychiformia, Anobiidse. Trypopitys sericeus. , Fig. 2,B. Serricornia, Elateridse. Hemicrefi'dius species.



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292 Psyche [December
(pL). The first visible pleurite in this case then is wholly formed of the third segment.
On the other hand an elater (Fig. 1, B) shows clear evi-
dence of the fusion commonly assumed. The second pleurite is very small (pL), but lies completely flush with the surface of the body and forms the actual articular surface facing the mete- pimeron. The suture is traceable dorsally, and continuous with the well marked fold between the second and third dorsal seg-
ments, but it soon becomes ambiguous, and I am not sure I have indicated its course correctly.
The obvious
conclusion from this is that the loss of the second pleurite has taken place twice independently (at least), and that its absence is not necessarily an indication of a homo- geneous series of "higher Coleoptera." In fact the presence of forms with a well marked second pleurite is known in both series (Omethes, for instance in the Serricorns besides the possibly degenerate Lampyridse; the early Dascyllidse in the Bostry- chif orm series).
Incidentally it appeared in this work that the common statements as to the number of free segments of the abdomen are not correct. In the elater only the last segment is free, while in Trypopitys the entire abdomen is ankylosed, even the sutures being obliterated, except the first. This is correctly stated in the descriptions of the genus, but not allowed for in any key I have seen.




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