REPRODUCTIVE IN THE CRICKET · Pakistani X Canadian Pakistani X Canadian Pakistani X Pakistani has characterized one of his races of the field cricket on the basis of its song. It

REPRODUCTIVE ISOLATION

IN THE HOUSE CRICKET

(ORTHOPTERA GRYLLIDAE)BY A. S. K GHOURI2 AND J. E. MCFARLANE

Entomology Department,

Macdonald College, P.Q., Canada

The existence of considerable geographic variation inthe morphology of the field cricket, Acheta assimilis Fab.,in North America has caused much confusion in its tax-onomy, and although the variants are now commonlygrouped into one species, the work of Fulton (1952) hasshown that this solution is no longer acceptable. Fultonfound that four "physiological races" of this insect whichare reproductively isolated exist in North Carolina, andthat although they show some average structural dif-ferences, intergradation between the races prevents theidentification of specimens on morphological grounds.The house cricket, Acheta domesticus (Linn.), is an

insect with a cosmopolitan distribution but, surprisingly,has been the subject of very little work, although itsavailability and ease .of rearing should have favoured itsuse as a laboratory animal. In the course of a study onthe physiology of the development of this insect (Ghouriand McFarlane, in preparation), we have compared a strainobtained from Pakistan, where it lives in the field, withone obtained in Canada and have found rather large dif-ferences in their rates of development, among other charac-teristics, although morphologically they are quite similar.This report is concerned with the results .of crossing ex-

This paper is based on a thesis submitted to McGill University byA. S. K. Ghouri in partial fulfillment of the requirements for the Ph.D.degree.Present address: Department of Plant Protection, Government of

Pakistan, Karachi.

3O

9] Ghouri and Mc’Farlane--Reproductive Isolation 31

periments between the two strains which show that theyare reproductively isolated.

METH,ODS

Eggs of the "Pakistani" strain were obtained from2emales collected near Usta Mohammad, West Pakistan,and sent to us by the Department o.f Plant Protection,Government of Pakistan, Karachi. The "Canadian" strainwas collected 2rom the basements of Macdonald College.Dr. A. B. Gurney of he U.S. National Museum, Washing-ton, D.C., very kindly confirmed the writers’ identifications.

Virgin females were obtained by segregating the sex.esas soon as the ovipositors could be detected in the hoppers.Single pairs were kept in two-pound .ointment jars andgroups of two or more pairs in one-gallon candy jars.

Crossing experiments were laid according to. the plansshown in Tables I and II. Oviposition dishes containingmoist sand were provided after 4 to 5 days and thereafteron alternate days until the female died. All eggs laid byeach female were counted and incubated. The spermathecaeo2 a number of 2emales were examined for sperms atvarious intervals ranging up to two months after the ex-periments were begun.

OBSERVATIONSThe results o2 the crossing experiments are shown in

Table I. The number of eggs laid in crosses within astrain, as shown in the table, does not represent the totalegg production o the females, but only the number ofeggs examined in order to establish that development wastaking place.The eggs laid in crosses between individuals of the same

strain and in the stock cultures usually hatched, whereaseggs laid in crosses between the strains invariably 2ailedto develop and died 4 to 6 days after incubation. Thespermathecae of 9 emales in the latter crosses, examinedat different intervals during the active oviposition periodof stock females of similar age, were devoid of sperms,

We are grateful to the Department of Plant Protection for supplyingeggs of the Pakistani strain.

32 Psyche [Marz.r

9] Ghouri and McFarlane--Reproductive Isola,tion 33

whereas the spermathecae .of females in the former crossesinvariably contained sperms.Although the ovaries of females in the crosses between

the strains, were apparently normal, containing largenumbers of eggs, these females laid a smaller number ofeggs than did the. females mated with males of the samestrain. A female of the Canadian strain generally with-held her eggs until the last days of her life. On the otherhand, Pakistani strain females mated to Canadian strainmales laid eggs comparatively earlier than the Pakistanifemales mated to Pakistani males.

Individuals of the two strains are not separable byany consistent structural differences (their morphologyhas, however, been far from adequately studied, and it isquite possible that obvious differences do exist); however,the strains may still be distinguished by average dif-ferences in appearance and behaviour. The average sizeof the body, ovipositor, the genital openings and sperma-tophores of the Canadian strain adults are larger thanthose ,of the Pakistani strain. The col,or of the Pakistanistrain adults varies from light brown with distinct mark-ings on the head to. very .dark brown with scarcely per-ceptible markings, whereas the adults, of the Canadianstrain are generally light brown and do not show as muchvariati.on (both light and dark Pakistani adults wereused in he crosses). Individuals of the Canadian strainare docile and seldom take to wing; on the other hand,adults .of the Pakistani strain are comparatively active,and have been observed to fly as many as 50 yards at astretch.

In spite of the mating sounds and courtship ,of the males(as described by Khalifa, 1950), neither the Pakistaninor the Canadian strain females showed interest in themales of the other strain. In the crosses, females of theCanadian strain often devoured the smaller males of thePakistani strain.

EFFECT OF REMOVAL OF TEGMINA OF MALES

The stridulation of the male cricket has commonly beenconsidered to be important in attracting the females.

34 Psyche [M,

Species differences in the sounds made by the male arefrequently detectable by the human ear, and Fulton (1952)

TABLE IICrossing experiments after removal of thetegmina of the males at the base, wings left intact.

Code1o.

CROSSSperms

Eggs Developed Found (+) in

(--) or Failed Spermathecato Develop (--) or Not

Found (m)

C1C2C3C4C5C6C7

Canadian X CanadianCanadian X CanadianCanadian X PakistaniCanadian X PakistaniPakistani X CanadianPakistani X CanadianPakistani X Pakistani

has characterized one of his races of the field cricket onthe basis of its song. It seemed possible that the matingsounds of the male were responsible for the unattractive-heSS of the males to the females of the other strain ofA. domesticus, although no differences between the strainswere apparent. The tegmina of the males of both strainswere accordingly severed at the base, leaving the wingsintact (the removal of the tegmina did not appear to aff.ectthe normal activities of the males), and crossing experi-ments were carried .out as shown in Table II.

Females mated with males of the same strain laidfertilized eggs and the spermatheca of one female wasfound to contain sperms, whereas females mated withmales of the .other strain laid eggs which invariably failedto develop, and no sperms were found in the spermathecaof the one female examined. Thus the stridulation of theparticipating male is not necessary for copulation (.or,more precisely, insemination), nor is it the sole discourage-ment to copulation in crosses between strains.

] Ghouri and McFarlane---Reproductive Isolation 35

DISCUSSION

These crossing experiments have shown that the Pakis-tani and Canadian strains of A. domesticus are repro-ductively isolated, and therefore are actually separatespecies, as defined biologically (Mayr et al., 1953).

In his crossing experiments with the races of A. as-similis, Fulton (1952) ound that while inseminationalways took place, eggs laid by the 2emales invariably2ailed to hatch. He concluded that hybrids were not pro-duced either because the eggs were not ertilized or be-cause the zygotes were not viable. With the two strainsof A. domesticus, however, reproductive isolation consistsin a failure of insemination to take place. It is possiblthat the strains of A. domesticus are not isolated at thegenetic level or at the level of ertilization that separatesthe races .of A. assimilis; however, attempts to achieveartificial insemination or fertilization have not been suc-cessful, due to technical difficulties.Although there can be little doubt that geographic isola-

tion has led to the differences between the Pakistani andCanadian -strains, it would be of interest to discover themechanism which is responsible for their isolation whenbrought together. Behavioural differences, in the broadsense, are obviously involved, and these may have to dowith actions during courtship or perhaps with the odourof the male. In A. assimilis, Fulton (1952) has foundthat two "races" may occupy the same habitat, and thepossibility that a more intensive study of both speciesgroups will show that geographic isolation is not essentialin speciation makes a study of the mechanism of isolationvery much worthwhile.The Pakistani and Canadian "strains" of A. domesticus

are two distinct species, but, as has been pointed out byGurney (1951) and Hubbell (1956) .or A. assimilis, nopractical purpose will be served, a the present stage of ourknowledge of these .crickets, by creating a new specificname. It will be desirable, however, to maintain a distinc-tion between the species by giving each a geographicdesignation.

Psyche,

LITERATURE CITED

FULTON, B. B.1952. Speciation in the field cricket. Evolution, 6: 283-295.

GHOUR, A. S. K. AND McFARLANE, J. E.1957. Observations on the development of crickets. Can. Ent. In press.

GURNEY, A. B.1951. The names of the field and house crickets. J. Econ. Ent., 44: 611.

HUBBELL, T. H.1956. Some aspects of geographic variation in insects. Ann. Bey.

Ent., 1 ’1-88.KHALIFA Z.

1950. Sexual behavior in Gryllus domesicus L. Behaviour, Leiden,2: 264-74.

MAYR, E., LINSLEY, E. G. AND USINGER, a. L.1953. Methods and principles of systematic zoology. New York. N.Y.,

McGraw-Hill Book Co.

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REPRODUCTIVE IN THE CRICKET · Pakistani X Canadian Pakistani X Canadian Pakistani X Pakistani has characterized one of his races of the field cricket on the basis of its song. It