In a recent experimental study Schmidtke (1955) failed to demonstrate a host-parasite relationship between Periplaneta americana and the haemosporidian Toxoplasma gondii Nicolle and Manceaux. This protozoan is a blood parasite in a rodent in North Africa (Kudo, 1954).
[XI. HELMINTHS]
Intestinal nematodes of the family Thelastomatidae have no apparent pathological effect on their cockroach hosts. Galeb (1878) has shown experimentally that oxyurids eat the same food as the host insect and that if one starves them, by withholding food from the host, the oxyurids die and disappear. In other words, these worms are not parasites, in the sense that we use the term in this paper, but commensals. Dobrovolny and Ackert (1934) stated that "all observations seemed to indicate that the health, fertility and activity of the heavily infested cockroaches were comparable with those of the non-parasitised specimens."
Very few papers have dealt with the ecology of the oxyurid parasites of cockroaches. According to Galeb (1878), usually one species of nematode is found in a single cockroach, but sometimes two species live together in the same host (e.g., in Blatta orientalis and Polyphaga aegyptiaca) where they compete for food. Galeb claimed that Hammerschmidtiella diesingi would replace Leidynema appendiculata; he observed that H. diesingi surpassed L. appendiculata in numbers and the latter became uncommon in the intestines of the cockroaches. On the other hand, Sobolev (1937) found that 48 percent of his oriental cockroaches were infected with both of the above species of nematodes. The average number of both species was 7.5, and the maximum number was 97; the mean number of H. diesingi was 5.1 and the maximum 64; the mean number of L. appendiculata was 2.4 and the maximum 33. More than 40 nematodes were found in each cockroach of 1.3 percent of those examined. These results apparently contradict Galeb's conclusions inasmuch as the number of each species in mixed infections was essentially the same as the number found in cockroaches infected by only one species (see pp. [195] and [197]). Dobrovolny and Ackert (1934) found that 29 percent of 222 Periplaneta americana contained both of the above species of nematodes; whereas 40 percent contained L. appendiculata only, and 21 percent contained H. diesingi only. The infestation ranged from 1 to 36 worms per cockroach with averages of 3.8 per male, 5.1 per female, and 2.7 per nymph.
The eggs of some helminths pass unharmed through the guts of cockroaches that serve as vectors of these ova and have no effect on the insect. However, helminths that are secondary parasites in cockroaches damage the insect to varying degrees depending upon the extent of the infection. Thus the larvae of Moniliformis moniliformis pass through the gut wall and some may become embedded in the fat tissue (Moore, 1946). First stage larvae of Oxyspirura mansoni also burrow through the midgut wall into the fat body; Sanders (1929) believed that Pycnoscelus surinamensis could be killed if at one time a sufficient number of migrating larvae of O. mansoni penetrated the cockroach's intestinal wall. Gongylonema neoplasticum migrates through the digestive tract and encysts in the muscles of the thorax and legs of the host (Fibiger and Ditlevsen, 1914). Infective larvae of Protospirura muricola, after hatching from ingested eggs, pass through the cockroach's gut wall and encyst mainly in the thorax, around the crop, and at the bases of the large muscles of the prothoracic legs (Foster and Johnson, 1939). It is probably generally true that nematodes which are secondary parasites in cockroaches do some damage to the host's intestinal tract at least, and they probably also damage other organs in which they may encyst.
Cockroach tissues may react defensively to infections by parasitic nematodes. For example, encysted third-stage larvae of Physaloptera turgida have been found enclosed in a thin, brown, chitinous substance that was undoubtedly deposited by the tissue of the cockroach (Alicata, 1937). Cysts of similar appearance have been found in cockroaches infected with Physaloptera rara, P. maxillaris, P. hispida (Petri, 1950; Hobmaier, 1941; Schell, 1952), and Gongylonema pulchrum (Schell, 1952a); in the latter species the deposit eventually completely surrounded the nematode larva which was killed and "chitinized." Apparently these pigmented cysts surround unhealthy or dead larvae and are secreted as a defensive mechanism by the host (Schell, 1952a). Oswald (1958) has reported finding similar pigmented cysts in Blatta orientalis and Periplaneta americana that were experimentally infected with Rictularia coloradensis.
Our classification of the helminths follows Hyman (1951, 1951a).
HELMINTHS FOR WHICH COCKROACHES SERVE AS PRIMARY HOSTS
Phylum ASCHELMINTHES