Parasitic insect

“ In fact, it becomes difficult to say where the line of parasitism should be drawn. I cannot, however, ignore all notice of the insect tormentors, whether strictly parasitic or not.
At least fifty different species have been regarded as playing the rôle of parasitism in man. ”

“ It will be seen from the foregoing discussion that the leading peculiarity of insect parasitism, at least in such groups as the Hymenoptera and Diptera, which are almost the only ones of value in controlling noxious insects, is the restriction of the parasitic habit to the sluggish larva and the specialization of the free adult for the purpose of disseminating the species and of placing the coming generation in intimate contact with the host. ”

“ In the case of insects having numerous food plants this method becomes less feasible, and here a scientific study of what may be termed entomological parasitology may prove useful. We must recognize that parasitic protection is never more than partial, but even a partial destruction of insect pests is of great value. ”

“ Whether they belong to one or another of these catagories, the predatory and parasitic insects play a regulating role that is useful and remarkable. When, on account of cultural conditions or climatic circumstances or other influences, the phytophagous species tends to increase beyond the average, it thus furnishes conditions eminently favorable to the multiplication of the parasitic species, and that in its turn causes the phytophagous form to decrease. ”

“ Now, since the parasites never attack more than one of the developmental stages of the insect—egg or larva, according to parasitic species—it results, from what precedes, that there will always be, at the period of egg-laying, existing individuals of the species which will escape them because they are in a developmental condition in which they are not pierced; and when the generation of parasites has passed, these individuals will remain unharmed and constitute the indispensable reserve for the perpetuation of the species. ”

“ All three of the earlier stages of butterflies—egg, larva, and chrysalis—are subject to attack by various parasitic insects which develop at the expense of the host. Such parasites are probably the most important check upon the increase of butterflies, and along with birds, mammals, and bacterial diseases, they help to keep up that balance of nature which in the long run maintains a surprising uniformity in the numbers of each kind of butterfly. ”

“ Hence, we can easily picture these ancient roaches scuttling up the tall trunks of the scaly lycopods, and shuffling in and out among the bases of the close-set leaf stems of the tree ferns, and we should expect to find an abundant infestation of them in the vegetational refuse matted on the ground. Insects of those days must have been comparatively free from enemies, for birds did not yet exist, and all that host of parasitic insects that attack other insects were not evolved until more recent times. ”

“ The parasitic Hymenoptera, are so called because they lay their eggs in other insects, that their offspring may have - 33 - fresh meat not only at birth, but so long as they need food, as the insect fed upon generally lives till the young parasite, which is working to disembowel it, is full-grown. ”

“ These parasites develop beneath the skin of the larva as footless grubs, which, at first, do not attack the vital organs, but later these organs are preyed upon, and the larva dies. A wasp-like insect which preys upon this larva well illustrates in its habits the crudeness of many instincts. The female will lay eight or ten eggs upon one caterpillar, but as the young parasitic grubs require a large amount of food, only one is able to mature and the others perish. ”

“ PART V.—PARASITIC DISEASES CHAPTER XXIV PARASITIC INSECTS AND MITES Parasitic insects are common causes of skin diseases in domestic animals. The diseased conditions of the skin, and the irritation that they may cause the animal, depend on the life history and habits of the parasite. ”

“ The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase ”

“ As examples of the so-called free parasitism, the autumnal flies (Tabanidæ) and Stomoxys may be cited. Although embracing but few strictly parasitic forms we have the following:—Œstridæ, Hippoboscidæ (with Melophagus) , and Nycteribiidæ. In regard to the maggots of Muscidæ and Sarcophagæ, some of them are parasitic on animals and man, whilst others are parasitic upon insects themselves. The larvæ of Conopidæ attack humble-bees internally. Those parasitic insects, properly so called, which, like certain of the crustaceans, are sometimes spoken of as epizoa, comprise three well-marked families. ”

“ The presence of these parasites finally exhausts the host, so that the sterile female bee dies prematurely.
41. Bee fungus.
As in the higher animals, bees are afflicted with parasitic worms which induce disease and sometimes death. The well-known hair worm, Gordius, is an insect parasite. The adult form is about the size of a slender knitting needle, and is seen in moist soil and in pools. It lays, according to Dr. Leidy, "millions of eggs connected together in long cords." The mi [Pg 47] croscopical, tadpole-shaped young penetrate into the bodies of insects frequenting damp localities. ”

“ Numbers of insects injurious to vegetation are held in check by parasites that destroy eggs and larvae; the chalcis flies are parasitic upon grain weevils, the destructive scale insects have for enemies various internal parasites. A common and troublesome parasite is the bot-fly, pest of horses. ”

“ And not only is practically the whole enormous group of moths and butterflies to be regarded as parasitic, but the same is true also of untold legions of plant-lice, scale-insects and beetles. Hyperparasitism, which may be regarded as a kind of permutation of parasitism, must also be mentioned in this connection, because it gives us a glimpse of the virgin fields which the holometabolic insects, owing to their peculiar method of development, are beginning to invade. ”

“ Paleontology seems to show very clearly the conditions that have favored the enormous development of parasitism among insects especially within comparatively recent times. Some of these conditions are:
1. The diminution in insect stature which occurred in the late Carboniferous and during the Permian and seems to have been originally in great part an adaptation to increased reproduction and dispersal. Other things equal, a small animal will, for very obvious reasons, become a parasite more easily than a large one. ”

“ A still larger number of parasites attack eggs of Insects, one or more individuals finding sufficient sustenance for growth and development inside another Insect's egg. As Insects have attacked Insects, so have parasites attacked parasites, and the phenomena called hyperparasitism have been developed. These cases of secondary parasitism, in which another {522}species attacks a primary parasite, are extremely numerous. It is also pretty certain that tertiary parasitism occurs, and Riley is of opinion that even quaternary destruction is not outside the range of probability. ”

“ Others, and this is the most common mode of parasitism, attack the insect in its larva state; others, in the pupa state, and still others in the perfect, or imago state. Dr. Leidy has shown that the wood-devouring species of beetle, Passalus cornutus, and some Myriopods, or "thousand legs," are, in some cases, tenanted by myriads of microscopic plants and worms which luxuriate in the alimentary canal, while the "caterpillar-fungus" attacks sickly caterpillars, filling out their bodies, and sending out shoots into the air, so that the insect looks as if transformed into a vegetable. ”

“ And the parasites themselves, do they lead unmolested lives? Are they the final arbiters of lire and death in the insect world? if you are fortunate sometime while study- ing aphids out-of'-doors, you may see a tiny black mite, no bigger than the small;est gnat, hovering about an in- fested plant or darting uncertainly from one leaf to another, with the air of searching for something but not knowing just where to look. You would probably suspect the intruder of being a parasite seeking a chance to place an egg in the body of an aphid ”

“ This is now completely overshadowed by the fact that so many of these parasitic forms are more than simple parasites, they are transmitters of other microscopic parasites which are pathogenic to man. Yet the importance of insects as parasites still remains and must be considered in a discussion of the relation of insects to the health of man. In taking up the subject we shall first consider some general features of the phenomenon of animal parasitism. ”

“ No thought seems to have been given to the conditions and the characteristics of the individual species of blood-sucking insects, which make disease transmission possible."
He points out that "in order to be a potential transmitter of human blood-parasites, an insect must be closely associated with man and normally have opportunity to suck his blood repeatedly. It is not sufficient that occasional specimens bite man, as, for example, is the case with forest mosquitoes. ”

“ A number of diseases in man and other animals are due to the presence of these Sporozoans, for they are all parasitic. Few if any animals are exempt from their attacks. They even attack other minute Protozoa. One hundred and fifty-seven species have been recorded as attacking insects, one hundred species attack birds, fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through the list. Ten have been recorded as attacking man. In some instances the parasite is always present in the host and some hosts may harbor several different species of Sporozoa. ”

“ Among other insects—such, for example, as certain ticks—a very complete form of female parasitism prevails; and while the male remains a complex, highly active, winged creature, the female, fastening itself into the flesh of some living animal and sucking its blood, has lost wings and all activity and power of locomotion, having become a mere distended bladder, which, when filled with eggs, bursts and ends a parasitic existence that has hardly been life. ”

“ In conclusion, it should be noted that the medical and entomological literature of the past few years abounds in suggestions, and in unsupported direct statements that various other diseases are insect-borne. Knab (1912) has well said "Since the discovery that certain blood-sucking insects are the secondary hosts of pathogenic parasites, nearly every insect that sucks blood, whether habitually or occasionally, has been suspected or considered a possible transmitter of disease. ”

“ More repulsive than the parasite female herself, because a yet further product of decay, it is yet only the scent of his mother’s boudoir that we smell in his hair. He is like to the bald patches and rotten wool on the back of a scabby sheep; which indeed indicate that, deep beneath the surface, a parasite insect is eating its way into the flesh, but which are not so much the cause of disease, as its final manifestation. ”

“ The insect parasites and predators of other insects in general comprise a class of insects that are most beneficial to us by reason of their large-scale destruction of species injurious to our crops. But, unfortunately, parasites as a class do not respect our classification of other creatures into harmful and useful species. ”

“ Diseases and their Prevention and Treatment.
In the tropics, as elsewhere, the diseases to which the traveller is liable are due to widely differing causes, but in hot countries climatic conditions and parasites play a predominant part. Insects are very frequently to blame for the transference of the latter from the sick to the sound, both insects which are not themselves parasitic on man, e.g., the mosquito and the tsetse fly, and those which make man their host, e.g., lice and fleas. ”

“ A somewhat similar fungus, Mucor mellitophorus (Fig. 41) , infests bees, filling the stomach with microscopical colorless spores, so as greatly to weaken the insect.
As there is a probability that many insects, parasites on the wild bees, may sooner or later afflict the Honey bee, and also to illustrate farther the complex nature of insect parasitism, we will for a moment look at some other bee parasites. ”

“ For the most part these insect parasites are small four-winged flies, although many of them are two-winged flies. In either case the life stages show a series of changes much like those of the butterflies themselves. Each parasite exists first as an egg, second as a larva, third as a pupa, and fourth as an adult fly. ”

“ In the densely populated countries of Europe, the losses occasioned by injurious insects are most severely felt, though from many causes, such as the greater abundance of their insect parasites, and the far greater care taken by the people to exterminate their insect enemies, they have not proved so destructive as in our own land. ”

“ Rose-bugs are also eaten greedily by domesticated fowls; and when they become exhausted and fall to the ground, or when they are about to lay their eggs, they are destroyed by moles, insects, and other animals, which lie in wait to seize them. Dr. Green informs us that a species of dragon-fly, or devil’s needle, devours them. He also says that an insect, which he calls the enemy of the cut-worm, probably the larva of a Carabus, or predaceous ground-beetle, preys on the grubs of the common dor-bug. ”