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204
Vol.13: no.4
APPARENT WILD HYBRIDS AMONG THE MEGATHYMIDiE
by Don B. Stallings, J. R. Turner, and Viola N. Stallings
As our studies of the Megathymidae have progressed, our belief that a good many of the species are of relatively recent origin has strengthened. If this is the situation, then when two species do meet and fly during the same time we should expect more wild hybrids than would be the case of older and more divergent species meeting, since there would have been less time for the newer species to strengthen and complete the "barrier of sterility or inviability."
The problem of recognizing wild hybrids in the field is much more difficult than is generally realized (see Remington, 1958), and for this reason a lot of natural hybridizing of Lepidoptera has been missed.
Agathymus evansi (H. A.Freeman) and Agathymus aryxna (Dyar) both occur in Arizona in Ramsey Canyon with their flight periods overlapping. Both we and H. A. Freeman are certain that wild hybrids occur but we have not as yet been able to satisfy ourselves which are the true hybrids and which are merely individual variants.
We are presently working on a situation in Mexico with a large number of variants within a relatively small area in which we have not as yet ruled out the possibility of hybridization and possibly introgression among the species involved.
At the time we described Agathymus carlsbadensis (Stallings & Turner, 1957), we mentioned that in the Guadeloupe Mts. near Nickle, Texas, we reared a single specimen ex-larva that was not A. carlsbadensis. Since then we have secured four more such specimens and now have two males and three females. We are reasonably sure that these specimens are Fi hybrids of Agathymus mar ice c? X carlsbadensis 9 . The reason that we feel pretty certain of the sex of each parent is that none of the larvae or pupae were secured from Agave lecheguilla Torr., which is the food-plant of A. marice; hence the female had to be A. carlsbadensis laying eggs on Agave parryi Engelm., the food-plant of A. carlsbadensis. Furthermore, this would be the expected cross, as generally A. marice flies after A. carlsbadensis and when the two do interbreed it should be a late female A. carlsbadensis and an early male A. marice. This cross was surprising to us due to the fact that A. marine is in a rather different species-group from that of A. carlsbadensis.
We now feel that Agave chisosensis Mueller does not occur in the Guadeloupe Mts. The plants in this area previously referred to as Agave chisosensis are, we believe, hybrids of Agave lecheguilla and Agave parryi (and here too, Agave lecheguilla is in a different subgenus from Agave parryi). As a matter of fact, wherever we have seen the so-called species Agave chisosensis we have found both Agave lecheguilla and a parryi-Wkz plant. In the Guadeloupe Mts. we find A. carlsbadensis and the hybrids (but
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Journal of the Lepidopterists* Society
205
AGATHYMUS HYBRIDS
Stallings & Turner
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206
Stallings & Turner: Hybrids
Vol.13: no.4
never A. marice) using the hybrid plant. As noted in the paper describing A. carlsbadensis, our observations indicated that the hybrid plant was not as compatible to the larvas as was true Agave parryi, resulting in a larger portion of the larvae on the hybrid plants not maturing.
An examination of the accompanying plate will indicate how the presumed hybrids seem to have modified characters of both parent species. The color of the spots of the hybrids is lighter than in A. carlsbadensis, but darker than in A. marice. We find little variation among the three male presumed hybrids or in the two females, the hatching period of all five specimens falls in the last part of the A. carlsbadensis flight and the first part of the A. marice
flight.
One of the strange things is that when we have shown these hybrids to lepidopterists not familiar with Megathymidae they immediately associate them with A. marice, while Freeman and we associated them with A. carlsbadensis. To us the genitalia seem to have more of the A. carlsbadensis characters, although the narrow indentation at the base of the female vaginal plate is certainly a character of A. marice.
The presence of hybrids in this area suggests some interesting possibili-tion. Remington (1958) states: "At least in Papilio, and perhaps in C alio samia, there is a most perplexing phenomenon: in many interspecific crosses the Fi hybrids are highly fertile in backcrosses, and completely sterile in Ft X Ft. pairings." If this same situation exists with these hybrids then we should expect some A. marice genes passing over to the A. carlsbadensis population. This in turn suggests that perhaps some of the "specific" characters of A. carlsbadensis, used for distinguishing it from A. neumoegeni (Edwards), are in fact variations caused by this introgression.
Freeman has discovered a similar situation in the Hueco Mts. of Texas in which there appear to be hybrids between A. marice (Barnes & McDun-nough) and A. judithce (Stallings & Turner).
References
Remington, Charles L., 1958. Genetics of lepidopterous populations. Proc. X. int. congress ent. 2: 787-805, 13 figs.
Stallings, Don B., & J. "R. Turner, 1957. Four new species of Megathymus. Ent. news 68: 1-17, 4 pis.
Caldwell, Kansas, U. S. A.
PLATE (p.205) : top, Agathymus carlsbadensis; middle, wild presumed hybrids; bottom, A. marice. All from near Nickle, Texas (see text). Left side males, right side females.