The Modification of the Eighth Sternite in Microdes (Lepidoptera).
[Read before the Nelson Philosophical Institute, 27th July, 1926; received by Editor, 30th July, 1926; issued separately, 10th August, 1927.]
It is not very unusual to find the eighth abdominal segment in the Lepidoptera modified to a greater or lesser degree. In the Micropterygidae the sternite of this segment may be absent (Sabatinca), or represented by a narrow strip only (Microyteryx); in Hepialidae it takes the form of a rather flat well chitinised plate lying beneath the vinculum, while in some Geometrids (Acidaliinae) the whole segment is irregular in shape and frequently armed with lateral processes (certa of Pierce), or the segment may be more normal but have a variously shaped more strongly chitinised median area (Eupitheciinae). These, and similar departures from the normal, are usually confined to the male sex and are almost certainly connected with the act of pairing.
In Microdes, a small genus confined to New Zealand and Australia, a very interesting structure is to be found. I have been able to examine the males of six species of the genus and it proved to be well developed in each. Pierce (the Genitalia of the British Geometridae) has described and figured this modified eighth sternite in the Eupitheciinae, the group to which Microdes belongs, and it appears that the structure assumes a considerable variety of forms; none of these, however, is quite similar to Microdes though Calliclystis approaches fairly near. As long ago as 1891 Dr. Buchanan White drew attention to the differences in the terminal segments of the males of Eupithecia. He described (entomologist, vol. 24, p. 129) more than thirty species, each showing marked differences in the “ultimate” (eighth) sternite, and, in some instances, in the “penultimate” (seventh) also.
The male genitalia of Microdes are normally completely withdrawn into the eighth segment which (see fig. 1), is greatly produced dorsally. The ventral portion of the eighth sternite is somewhat flattened and bears a forcipate structure, the base of which rests on the anterior margin, while the tips of the prongs project just beyond the posterior edge of the sclerite. This organ, for which I suggest the name “lyra,” is apparently a development of the sternite itself, being formed simply by stronger chitinisation. It will be seen from the figures, which are of the same magnification, that the lyra provides a very good specific character, at least for those forms which have been available for examination.
In what manner the lyra functions it is difficult to imagine. Its position on the eighth sternite would seem to prevent it from participation in conjunction with the extruded parts of the ninth segment; nor do the genitalia of the female show any unusual external features which might suggest adaptation to the male structure.
Microdes epicryptis Meyr. and M. quadristrigata Walk. are New Zealand species; the others are from Australia. I have pleasure in expressing my best thanks to Mr. G. V. Hudson for material of M. epicryptis and to Dr. A. J. Turner and Mr. G. Lyell for the Australian insects.
Fig. 1.—Microdes squammulata Guen. Obliquely lateral view of male genitalia in normal position. 8, eight segment; h, harpes; 1, lyra.
Fig. 2.—M. squammulata Guen. Eighth sternite, ventral view.
Fig. 3.—M. diplodonta Turn. Eighth sternite, ventral view.
Fig. 4.—M. oreochares Turn. Eighth sternite, ventral view.
Fig. 5.—M. villosata Guen. Eighth sternite, ventral view.
Fig. 6.—M. quadristrigata Walk. Eighth sternite, ventral view.
Fig. 7.—M. epicryptis Meyr. Eighth sternite, ventral view.
Fig. 8.—M. villosata Guen. Apical segments of female. 7, 8 and 9, seventh, eighth and ninth segments; co, copulatory opening.