VESCICATING INSECTS AND THOSE POSSESSING OTHER POISONS IN THEIR BLOOD PLASMA
We have seen that certain forms, for example, the poisonous spiders, not only secrete a toxine in their poison glands, but that such a substance may be extracted from other parts of their body, or even their eggs.
There are many insects which likewise possess a poisonous blood plasma.
Such forms have been well designated by Taschenberg as _cryptotoxic_ (???pt?s = hidden). We shall consider a few representative forms.
[Ill.u.s.tration: 42_a_. Blister beetle.]
[Ill.u.s.tration: 42_b_. An American blister beetle. Meloe angusticollis.
Photograph by M. V. S.]
THE BLISTER BEETLES--Foremost among the cryptotoxic insects are the _Meloidae_ or "blister beetles," to which the well-known "Spanish fly"
(fig. 42_a_), formerly very generally used in medical practice, belongs.
The vescicating property is due to the presence in the blood plasma of a peculiar, volatile, crystalline substance known as _cantharidin_, which is especially abundant in the reproductive organs of the beetle.
According to Kobert, the amount of this varies in different species from .4 or .5% to 2.57% of the dry weight of the beetle.
While blister beetles have been especially used for external application, they are also at times used internally as a stimulant and a diuretic. The powder or extract was formerly much in vogue as an aphrodisiac, and formed the essential const.i.tuent of various philters, or "love powders". It is now known that its effects on the reproductive organs appear primarily after the kidneys have been affected to such an extent as to endanger life, and that many cases of fatal poison have been due to its ignorant use.
There are many cases on record of poisoning and death due to internal use, and in some instances from merely external application. There are not rarely cases of poisoning of cattle from feeding on herbage bearing a large number of the beetles and authentic cases are known of human beings who have been poisoned by eating the flesh of such cattle. Kobert states that the beetles are not poisonous to birds but that the flesh of birds which have fed on them is poisonous to man, and that if the flesh of chickens or frogs which have fed on the cantharidin be fed to cats it causes in them the same symptoms as does the cantharidin.
Treatment of cases of cantharidin poison is a matter for a skilled physician. Until he can be obtained, emetics should be administered and these should be followed by white of egg in water. Oils should be avoided, as they hasten the absorption of the poison.
OTHER CRYPTOTOXIC INSECTS--Though the blister beetles are the best known of the insects with poisonous blood plasma, various others have been reported and we shall refer to a few of the best authenticated.
One of the most famous is the Chrysomelid beetle, _Diamphidia simplex_, the body fluids of whose larvae are used by certain South African bushmen as an arrow poison. Its action is due to the presence of a toxalb.u.min which exerts a haemolytic action on the blood, and produces inflammation of the subcutaneous connective tissue and mucous membranes. Death results from general paralysis. Krause (1907) has surmised that the active principle may be a bacterial toxin arising from decomposition of the tissues of the larva, but he presents no support of this view and it is opposed by all the available evidence.
In China, a bug, _Heuchis sanguinea_, belonging to the family Cicadidae, is used like the Meloidae, to produce blistering, and often causes poisoning. It has been a.s.sumed that its vescicating properties are due to cantharidin, but the presence of this substance has not been demonstrated.
Certain Aphididae contain a strongly irritating substance which produces, not merely on mucous membranes but on outer skin, a characteristic inflammation.
It has been frequently reported that the larvae of the European cabbage b.u.t.terfly, _Pieris bra.s.sicae_, accidentally eaten by cows, horses, ducks, and other domestic animals, cause severe colic, attempts to vomit, paralysis of the hind legs, salivation, and stomat.i.tis. On _postmortem_ there are to be found haemorrhagic gastro-enteritis, splenitis, and nephritis. Kobert has recently investigated the subject and has found a poisonous substance in the blood of not only the larvae but also the pupae.
FOOTNOTES:
[A] This is diametrically opposed to the findings of Bordas (1905) in the case of the European _Latrodectus 13-guttatus_, whose glands are "much larger than those of other spiders." From a considerable comparative study, we should also unhesitatingly make this statement regarding the glands of our American species, _L. mactans_.
[B] Dr. E. H. Coleman (Kellogg, 1915) has demonstrated its virulence by a series of experiments comparable with those of Kobert.
[C] According to Stiles, the species occurring in the Northwest which is commonly identified as _D. venustus_ should be called _D. andersoni_ (see footnote, chapter 12).
[D] It should be remembered that in all the higher Hymenoptera the first abdominal segment is fused with the thorax and that what is apparently the sixth segment is, in reality, the seventh.
CHAPTER III
PARASITIC ARTHROPODA AFFECTING MAN
The relation of insects to man as simple parasites has long been studied, and until very recent years the bulk of the literature of medical entomology referred to this phase of the subject. 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.
Parasitism is an adaptation which has originated very often among living organisms and in widely separated groups. It would seem simple to define what is meant by a "parasite" but, in reality, the term is not easily limited. It is often stated that a parasite is "An organism which lives at the expense of another," but this definition is applicable to a predatory species or, in its broadest sense, to all organisms. For our purpose we may say with Braun: "A parasite is an organism which, for the purpose of obtaining food, takes up its abode, temporarily or permanently, on or within another living organism".
Thus, parasitism is a phase of the broad biological phenomenon of _symbiosis_, or living together of organisms. It is distinguished from _mutualism_, or symbiosis in the narrow sense, by the fact that only one party to the arrangement obtains any advantage, while the other is to a greater or less extent injured.
Of parasites we may distinguish on the basis of their location on or in the host, _ecto-parasites_, which live outside of the body; and _endo-parasites_, which live within the body. On account of their method of breathing the parasitic arthropods belong almost exclusively to the first of these groups.
On the basis of relation to their host, we find _temporary parasites_, those which seek the host only occasionally, to obtain food; and the _stationary_ or _permanent_ parasites which, at least during certain stages, do not leave their host.
_Facultative parasites_ are forms which are not normally parasitic, but which, when accidentally ingested, or otherwise brought into the body, are able to exist for a greater or less period of time in their unusual environment. These are generally called in the medical literature "pseudoparasites" but the term is an unfortunate one.
We shall now take up the different groups of arthropods, discussing the more important of the parasitic forms attacking man. The systematic relationship of these forms, and key for determining important species will be found in Chapter XII.
ACARINA OR MITES
The ACARINA, or _mites_, form a fairly natural group of arachnids, characterized, in general, by a sac-like, unsegmented body which is generally fused with the cephalothorax. The mouth-parts have been united to form a beak or rostrum.
The representatives of this group undergo a marked metamorphosis.
Commonly, the larvae on hatching from the egg, possess but three pairs of legs, and hence are called _hexapod larvae_. After a molt, they transform into nymphs which, like the adult, have four pairs of legs and are called _octopod nymphs_. These after a period of growth, molt one or more times and, acquiring external s.e.xual organs, become adult.
Most of the mites are free-living, but there are many parasitic species and as these have originated in widely separated families, the Acarina form an especially favorable group for study of the origin of parasitism. Such a study has been made by Ewing (1911), who has reached the following conclusions:
"We have strong evidence indicating that the parasitic habit has originated independently at least eleven times in the phylogeny of the Ararina. Among the zoophagous parasites, the parasitic habit has been developed from three different types of free-living Acarina: (a) predaceous forms, (b) scavengers, (c) forms living upon the juices of plants."
Ewing also showed that among the living forms of Acarina we can trace out all the stages of advancing parasitism, semiparasitism, facultative parasitism, even to the fixed and permanent type, and finally to endoparasitism.
Of the many parasitic forms, there are several species which are serious parasites of man and we shall consider the more important of these.
Infestation by mites is technically known as _acariasis_.
[Ill.u.s.tration: 43. Effect of the harvest mites on the skin of man.
Photograph by J. C. Bradley.]
The Trombidiidae, or Harvest Mites
In many parts of this country it is impossible for a visitor to go into the fields and, particularly, into berry patches and among tall weeds and gra.s.s in the summer or early fall without being affected by an intolerable itching, which is followed, later, by a breaking out of wheals, or papules, surrounded by a bright red or violaceous aureola, (fig. 43). It is often regarded as a urticaria or eczema, produced by change of climate, an error in diet, or some condition of general health.
Sooner or later, the victim finds that it is due to none of these, but to the attacks of an almost microscopic red mite, usually called "jigger" or "chigger" in this country. As the term "chigger" is applied to one of the true fleas, _Dermatophilus penetrans_, of the tropics, these forms are more correctly known as "harvest mites." Natives of an infested region may be so immune or accustomed to its attacks as to be unaware of its presence, though such immunity is by no means possessed by all who have been long exposed to the annoyance.
[Ill.u.s.tration: 44. Harvest mites. (Larvae of Trombidium). After C. V.
Riley.]
The harvest mites, or chiggers, attacking man are larval forms, possessing three pairs of legs (fig. 44). Their systematic position was at first unknown and they were cla.s.sed under a special genus _Leptus_, a name which is very commonly still retained in the medical literature. It is now known that they are the larval forms of various species of the genus _Trombidium_, a group of predaceous forms, the adults of which feed primarily on insects and their eggs. In this country the species best known are those to be found late in summer, as larvae at the base of the wings of houseflies or gra.s.shoppers.
There is much uncertainty as to the species of the larvae attacking man but it is clear that several are implicated. Bruyant has shown that in France the larvae _Trombidium inapinatum_ and _Trombidium holosericeum_ are those most frequently found. The habit of attacking man is abnormal and the larvae die after entering the skin. Normally they are parasitic on various insects.