Certain organisms which live in cockroaches appear to have no effect on the vitality of the host even though the tissues of the host are invaded. Gregarines may penetrate the intestinal wall of the cockroach without seeming to injure the host. Fungi of the genus Herpomyces invade the cuticle of cockroaches producing pathological changes; yet the insects' behavior is apparently unaffected (see p. 129). We consider these organisms to be parasites because the host's tissues are invaded and, as far as we know, no benefit to the host results.
In the literature certain insects have been considered to be either parasites or predators or both. Among these are the ensign wasps (Evaniidae), whose larvae feed on the eggs of cockroaches within the oötheca, and the ampulicid wasps, which capture, paralyze, and store in their nests (as food for their larvae) nymphs and adults of cockroaches. Clausen (1940) claimed that the evaniid Zeuxevania splendidula is a true egg parasite when it destroys the first egg in a cockroach oötheca; but after the wasp larva molts and proceeds to devour the other eggs, he considered it to be a predator. Clausen's definition of an entomophagous parasite is an insect that in its larval stage develops either internally or externally upon a single host which is eventually killed; with few exceptions the adults are free-living and their food is usually different from that of the larvae. A predatory insect, by Clausen's definition, is principally free-living in the larval as well as adult stage, kills the host immediately by direct attack, and requires a number of victims to reach maturity; the predator is of greater size than the prey, and the food sources of the adults and immature stages are frequently the same.
It is apparent, as Clausen and other writers have pointed out, that there are instances of a particular species showing characteristics which fit both the definitions for predator and parasite. Thus, among the evaniids one wasp larva destroys all the eggs in an oötheca, but in spite of this the larva has more of the characteristics of a parasite than of a predator; the adult wasp does not utilize the same food as the larva (adults have been taken on flowers and on honeydew from scale insects). It is questionable whether the evaniid larva kills the cockroach egg outright. The wasp larva, being restricted to the inside of the oötheca, is not free-living. Probably the only criterion by which the evaniid could be judged to be a predator, by Clausen's definition, is that more than a single egg is devoured by the maturing wasp larva.
Among the other wasp parasites (Encyrtidae, Eulophidae, Eupelmidae) of cockroach eggs many individuals develop in a single oötheca. When a hundred or more wasps emerge from an oötheca which contained less than 20 eggs, it is obvious that a single cockroach egg supported more than one wasp, yet it is possible that one particular wasp larva may have fed upon more than one cockroach egg before becoming an adult. We consider all entomophagous wasps that develop in cockroach oöthecae to be parasites rather than predators.
On the other hand, even though Anastatus floridanus, A. tenuipes, and Tetrastichus hagenowii are egg parasites as larvae, the adult females are, in a sense, predators when they sometimes eat part of the cockroach egg that oozes through the oviposition puncture (Roth and Willis, 1954a, 1954b). Williams (1929) has seen the female of Ampulex canaliculata imbibe blood that oozed from the cut ends of the cockroach's antennae after she had clipped them off before leading the prey to her nest. Yet despite this evident predatism on the part of the adult, the larva feeds as a parasite on the stored cockroaches in accordance with Sweetman's (1936) (though not Clausen's 1940) definition of parasitism, which is "that form of symbiosis in which one symbiont lives in or on the host organism and feeds at its expense during the whole of either the immature or mature feeding stage." The ampulicid larva, as the evaniid, is not free-living and does not kill the host immediately by direct attack, even though it may require more than one victim to reach maturity. Thus, within one individual both parasitic and predatory behavior are operant during different stages of its life history.
With the above discussion in mind we have summarized below the various biotic associations of cockroaches. Only a few examples are given for each section, but all organisms with similar habits presumably would be classified in the same categories.
Class A. Associations in which cockroaches serve as hosts, vectors, or prey for other organisms.
Type I. Obligate associates. Animals and plants that normally develop only on or in the cockroach; in general, these organisms depend entirely upon the cockroach for survival.
Group 1. Mutuals (symbiotes or symbionts of authors).
(a) Bacteria-like organisms (bacteroids which are found in the fat body of all cockroaches that have been examined; p. [96]).
(b) Bacteria (wood-digesting forms in Panesthia, and possibly certain bacteria in the intestines, of other cockroaches; p. [100]).
(c) Protozoa (several genera and species found in Cryptocercus; p. [101]).
Group 2. Commensals.
(a) Protozoa (Nyctotherus, Herpetomonas, Lophomonas, etc.; p. [172]).
(b) Nematodes (Thelastomatidae; p. [193]).
Group 3. Parasites.
(a) Fungi (Laboulbeniales; p. [134]).
(b) Protozoa (gregarines, Plistophora, etc.; p. [181]).
(c) Helminths.
(1) Primary parasites (mermithids and gordian worms; p. [201]).
(2) Secondary parasites (Gongylonema neoplasticum, Oxyuris mansoni, Moniliformis spp.; p. [206]).
(d) Arthropods.
(1) Mites (Pimeliaphilus podapolipophagus; p. [219]).
(2) Insects (larvae of ripiphorids, evaniids, and ampulicids; p. [231]).
Type II. Facultative associates. Animals and plants that prey on cockroaches or are incidentally or accidentally picked up by the cockroach, but which can survive or propagate readily on some other host or prey. Steinhaus (1946) emphasized the importance of the environment in determining the type of microbial flora associated with the cockroach which may carry one type of flora in an area which is exposed to filth and a different type in other areas. Because many of these organisms survive passage through or on the cockroach, the blattid may act as a vector of these animals and plants.
Group 1. Commensals.
(a) Viruses (strains of poliomyelitis virus; p. [103]).
(b) Bacteria (Enterobacteriaceae, Pseudomonadaceae, Micrococcaceae, etc.; p. [111]).
(c) Fungi (Aspergillus; p. [130]).
(d) Protozoa (Iodamoeba, Dobellina, and cysts of Entamoeba coli and Entamoeba histolytica; p. [179]).
(e) Helminths (cysts of various helminths parasitic in vertebrates; p. [208]).
(f) Arthropods.
(1) Mites (Tyrophagus lintneri; p. [218]).
Group 2. Parasites.
(a) Bacteria (Serratia marcescens; p. [117]).
(b) Helminths (Protospirura spp.; p. [206]).
(c) Arthropods.
(1) Mites (Locustacarus sp.; p. [219]).
(2) Insects (Melittobia chalybii; p. [248]).
Group 3. Predators, active.
(a) Arthropods.
(1) Spiders (p. [214]).
(2) Scorpions (p. [212]).
(3) Centipedes (p. [222]).
(4) Mites (Rhizoglyphus tarsalus; p. [218]).
(5) Insects (dermestids, reduviids, and on occasion adult females of Tetrastichus, Anastatus, Ampulex; p. [234]).
(b) Vertebrates.
(1) Amphibia (p. [269]).
(2) Reptilia (p. [272]).
(3) Aves (p. [276]).
(4) Mammalia (p. [283]).
Group 4. Predators, passive: Pitcher plants (p. [154]).
Class B. Associations in which cockroaches serve as commensals or predators.
Group 1. Commensal cockroaches.
(a) Associates of social insects (Attaphila spp., etc.; p. [315]).
(b) Obscure associates (p. [316]).
Group 2. Predatory cockroaches.
(a) Interspecies predators (p. [319]).
(b) Intraspecies predators (p. [322]).
Class C. Associations of cockroaches with other cockroaches.
Group 1. Intraspecies associations.
(a) Familial associations (p. [325]).
(b) Other conspecific associations (aggregations and fighting) (p. [336]).
Group 2. Interspecies associations.
(a) Compatible associations (p. [337]).
(b) Antagonistic associations (p. [329]).
Class D. Ecological associations of cockroaches with higher plants.
Group 1. Benign associations (p. [139]).
Group 2. Associations detrimental to plants (p. [162]).