It is apparent from the numbers of predators reported herein that many animals are not deterred by the odorous secretions of cockroaches; these secretions, because they may seem repugnant to man, are often claimed to be repellent to predators. However, Cott (1940) points out that "There are many instances in which protective devices and associated warning colours are known to be ineffectual against certain enemies. But this does not necessarily imply that they are not on the whole beneficial to the species attacked." Certain cockroach secretions may well be repellent to many predators, but as this is a purely negative aspect of the predator-prey relationship little thus far has been observed or published. Potential prey that successfully defends itself against attack is never found in a predator's stomach.
Cockroaches have a variety of glands which secrete odorous materials. Certain secretions, produced by tergal or dorsal glands in males, are involved in sexual behavior; the females feed on the secretion from these glands prior to copulating (Roth and Willis, 1954). However, other secretions which are produced by both sexes are ejected or given off when the insect is disturbed; undoubtedly these are defensive weapons that are used against predators. Very few experiments or observations are on record to show how effective these secretions may be in protecting the cockroach. Although the morphology of some of the glands has been described, relatively little is known about the chemistry of their secretions.
Many species of Australian cockroaches have been reported to emit "disgusting" odors, though the glands producing these secretions have not been described, nor is the chemistry of the compounds known. Cosmozosteria lateralis exposed two orange-red spots on the abdomen while emitting a pungent odor which deterred a collector from capturing it (Shelford, 1912). Another Australian species, Platyzosteria castanea, when disturbed on barren ground tilts forward on the vertex and straddles out the posterior legs, supporting itself in a vertical position on the head and tarsi; in assuming this attitude it will squirt a foetid fluid as a fine spray for a distance of 6 or 7 inches (Shaw, 1914). Spencer (1892) mentions the pungent odor given off by a cockroach which had been accidentally cut in two. Rageau (1956) stated that in the New Hebrides and New Caledonia Cutilia nitida emits, when disturbed, a corrosive liquid with an extremely disagreeable odor.
The adults of Eurycotis floridana emit an odorous fluid when seized (Rehn and Hebard, 1905). The fluid, which may irritate sensitive skin areas, may be ejected as a spray for a distance of several inches. This secretion has been identified as 2-hexenal (Roth et al., 1956), and the ventral abdominal glands which produce it have been described (Stay, 1957). Eisner (personal communication, 1958) has found that the toad Bufo marinus and the frog Rana pipiens invariably spit out adults of E. floridana which they have seized. The odor of 2-hexenal was strongly apparent after these attacks, and the insect was never damaged. However, the lizard Anolis equestris seized and crushed E. floridana before releasing its hold and dropping the insect 5 to 10 minutes later. The blue jay Cyanocitta cristata readily attacked adults of E. floridana and killed them but did not eat the insects until after the odor had dissipated; however, the bird carried nymphs of E. floridana to its perch and ate them. Nymphs of this species do not secrete 2-hexenal (Roth et al., 1956). Recently, 2-hexenal has been tested for its antibacterial activity and has been found to be active against seven species of pathogenic bacteria (Valcurone and Baggini, 1957). Eurycotis decipiens from Trinidad also ejects a fluid which may produce toxic symptoms such as vertigo and nausea (Bunting in Roth and Willis, 1957a).
Large reservoirs of glands similar in appearance and position to those of Eurycotis floridana are present in the adults of both sexes of Neostylopyga rhombifolia and Platyzosteria novae seelandiae. Walker (1904) and Longstaff (in Shelford, 1912) noted that the latter species had a strong odor. Roth (unpublished data, 1957) found that the secretion of P. novae seelandiae when ejected is grayish or milky in color. In the reservoirs of the ventral gland of this insect the secretion is a milky liquid containing floating greenish globules. Both infrared and mass spectrographic analyses show that the secretion is a mixture containing 2-hexenal, the aldehyde that is found in E. floridana. Eisner (personal communication, 1958) observed that the lizard Anolis carolinensis immediately released Neostylopyga rhombifolia without injury, but that Bufo marinus, Anolis equistris, and Cyanocitta cristata ate the insect despite the secretion; several unidentified spiders and the ant Pogonomyrmex badius were not repelled by the secretion of N. rhombifolia.
Dorsal and ventral glands have been found in both sexes of Blatta orientalis and Periplaneta americana (Minchin, 1888, 1890; Kul'vets, 1898; Oettinger, 1906; Harrison, 1906; Liang, 1956). The ventral glands are found in the same general region as those of Eurycotis. We have also found similar ventrally located glands in both Periplaneta australasiae, and P. brunnea. The reservoirs which store the secretion of the ventral glands are smaller in Blatta and Periplaneta spp. than those found in Eurycotis, Neostylopyga, or Platyzosteria.
In Blatta orientalis the dorsal glands can be everted by pressure on the abdomen; the secretion in these glands, according to Haase (1889), has the typical oriental cockroach odor. Although the dorsal glands of the oriental cockroach are usually given a defensive role (Haase, 1889, 1889a; Kul'vets, 1898; Oettinger, 1906; Konček, 1924), the functions of secretions of these nonepigamic dorsal glands and the ventral glands are still open to question. It is possible that some of the odors produced by cockroaches have functions other than defense or sex attraction. For example, Ledoux (1945) showed that the species odor is largely responsible for the gregarious behavior shown by Blatta orientalis and Blattella germanica. The olfactory stimulus acts over a short distance only, and the source of this odor in the insect is unknown. By washing Blattella germanica in warm chloroform Dusham (1918) extracted a wax which had the odor of the German cockroach. However, there is no evidence to show that cockroaches respond to the same cockroach odors that are detected by man.
Certain cockroaches have recently been found to have odorous secretions which are produced in tracheal glands. In Diploptera punctata the tracheae leading to the second abdominal spiracles of nymphs and adults are modified into odoriferous glands which produce a mixture of 2-ethyl-1,4-benzoquinone; 2-methyl-1,4-benzoquinone; and para benzoquinone; this material is ejected as a means of defense. The offensive odor emitted by adults and nymphs of Leucophaea maderae also issues from the second abdominal spiracles (Roth and Stay, 1958).
Diploptera is capable of ejecting its quinones from either its right or left tracheal gland according to which side of the insect is attacked (pl. [36], A-B). Eisner (1958) found that the secretion repelled the ant Pogonomyrmex badius (Latreille) (pl. [36], C) and the beetle Galerita janus Fabricius when they attacked the cockroach. The spider Lycosa helluo Walckenaer was repelled by large nymphs and adults of D. punctata but young nymphs were usually eaten promptly (Eisner, 1958).