Encyclopedia of Parasitology

2016 Edition
| Editors: Heinz Mehlhorn

Insects

  • Günter Schaub
  • Heinz Mehlhorn
Reference work entry
DOI: https://doi.org/10.1007/978-3-662-43978-4_1610

Classification

Class of  Arthropoda.

General Information

The Insecta are the largest group of animals with respect to the number of species (~773,000) and to individuals. The classification is based on the original occurrence ( Pterygota) or absence ( Apterygota) of wings. The further developed subclass Pterygota includes all important parasitic species; some of them, however, have apparently lost their primary wings during adaptation to parasitism (e.g.,  fleas). Insects may act as ectoparasites when sucking blood on the surface of their hosts (e.g.,  mosquitoes), or may even become endoparasites when entering the skin, and the intestinal and/or respiratory tracts in a variety of hosts (e.g., flies leading to myiasis;   Dermatobia hominis ). In addition to this method of parasitism the insects may be directly or indirectly involved in the life cycles of a large number of parasites. These insects are encountered:
  • as true intermediate or final hosts of important parasites of humans and animals (e.g., Protozoans,  Platyhelminthes,  Nematodes)

  • as true vectors of pathogens (including an inner production phase) such as bacteriae, rickettsiae, and viruses (e.g., fleas, body  lice)

  • as mechanical vectors of some parasites (transporting parasitic stages via mouthparts) (e.g., Entamoeba-, Giardia cysts)

The body organization of parasitic insects is very often closely adapted for its peculiar way of life and the special needs of feeding. However, the following basic features are commonly recognized:
  • The body shows a clear segmentation into the head (caput), breast (thorax), and trunk (abdomen), each part consisting of several specific segments (visible from outside or not).

  • The chitinous exoskeleton is regularly molted during growth (Fig. 1).

  • The caput, the segments of which form a strong capsule, is endowed with a pair of dorsal, segmented antennae (Fig. 2) and 3 pairs of ventral mouthparts (mandibles, maxillae 1 and 2), the latter being strongly adapted for their special way of feeding. In general, eyes are compound and located close to the basis of the antennae; the eyes are mostly composed of numerous single ommatidia, in rare cases (e.g., fleas) only one or a few ommatidia are present.

  • The thorax always consists of 3 segments (pro-, meso- and metathorax), which each bear ventrally a pair of legs (e.g., the name hexapoda means six feet). These legs are segmented and composed of 5 distinct parts (coxa, trochanter, femur, tibia, and tarsus); the tarsus comprises several single segments and is equipped with species-specific holdfast systems, claws, etc.

  • The meso- and/or metathorax may form typical membranous wings (formed by the  integument) which are moved by strong inner (mostly indirect) muscular systems. Wings, however, are reduced secondarily in some groups (e.g., fleas, bedbugs, lice).

  • The abdominal segments form no ventral extremities except for some specific copulatory appendages. Inside the abdomen important systems of the insects are found (gonads, heart, excretory system, Malpighian tubules, etc.).

  • Respiration of insects proceeds using a large, widely branched tracheal system reaching up to the surface of single cells.

  • The inner side of the gut of many species is lined by a single or several chitinous  peritrophic membranes (Fig. 3), which are a considerable obstacle for some parasites on their way to the intestinal wall and to the body cavity.

  • In general, the fertilized eggs of insects show a total, superficial cleavage, which in the Pterygota is followed by 2 different types of larval development. Hemimetabolic development proceeds as constant  metamorphosis via molting larvae (also named nymphs), which resemble the final adult stage and become sexually mature after the last molt. Holometabolic development is characterized by an additional  pupa, which gives rise by molt to the sexually mature adult ( Imago). The pupa may be completely inactive (e.g., fleas, flies) or may be motile (as in  mosquitoes), but never feeds.

Insects, Fig. 1

Diagrammatic representation of the reproductive system of a male dog flea ( Ctenocephalides canis) (AH accessory glands, DE ductus ejaculatorius, HO testis, NH “epidydymis” (vas efferens), PE penis)

Insects, Fig. 2

Diagrammatic representation of a longitudinal section through a female flea (  Xenopsylla cheopis ) (DGA dorsal head ganglion (superesophageal), E esophagus, GO genital opening, H heart, HI hindgut, MA Malpighian tubes, MI midgut, OV ovarioles, P pharynx, PR proventriculus, PY pygidium, R rectum, SG salivary gland, VGA ventral head ganglion (subesophageal), VN ventral nerve chord)

Insects, Fig. 3

Diagrammatic representation of the reproductive system of female insects (AC accessory glands, CO common oviduct, GL spermathecal gland, L ligament, LO lateral oviduct, OL ovariole, OV ovary, S sphincter, SP spermatheca, VG vagina)

System

Considering exclusively the parasitic groups, the following summarized classification is in general accepted:

Reproduction

Most parasitic insects are  oviparous. In some genera (e.g.,   Oestrus ,   Sarcophaga ) eggs are retained until larvae are ready for hatching ( Ovoviviparous). A few species (e.g., some   Musca spp.,   Glossina spp.) are  larviparous, laying more or less highly developed larval  instars. Even  pupiparity can be found (e.g., Melophagus ovinus) when immobile, fully developed larval instars pupate during deposition. The ontogeny of parasitic insects occurs either as  hemimetabolous development or  holometabolous development.

Reproductive Organs

Insects are typically  dioecious, this being regulated by different sex chromosomes.  Sexual dimorphism becomes evident with the development of the abdominal copulatory appendages. In general the female genital pore is situated ventrally at the posterior margin of the 8th segment (sternite), whereas the male pore is situated along the ventral midline of the 9th, which also forms the copulatory appendages.

The male system (e.g., fleas; Fig. 4) is situated dorsal to the intestine. It consists of 2 testes, each with 1 to several sperm-producing follicles, 2 vasa deferentia (each with an enlargement, the seminal vesicle), 2 accessory glands which join the vasa deferentia, a single ejaculatory duct formed by fusion of the vasa deferentia, a penis which communicates with the exterior via the genital pore, and several species-specific external copulatory appendages.
Insects, Fig. 4

Diagrammatic representation of a typical insect  cuticle (BL basal lamina, CE cement layer, EC epicuticle, EN endocuticle, EP epidermis, EX exocuticle, GC gland cell, GD, gland ductus, HM hemocyte, OE oenocyte, P pore channel, PO polyphenol layer, W wax layer)

The female system (Figs. 5 and 6) has 2 ovaries. These are composed of a species-specific number of tube-like ovarioles, which differ with respect to the location of the vitelline cells, but which all include a string of  oocytes, of which the one nearest the oviduct is the first to mature. In the posterior part the ovarioles unite to form the calyx, which opens into the lateral oviduct. The female system also has 2 lateral oviducts, a common central oviduct, a common vagina often leading into an exterior ovipositor, a single spermatotheca (seminal receptacle, which opens into the vagina as does a spermathecal gland) and paired accessory glands that empty into the vagina.
Insects, Fig. 5

Transmission electron micrographs of insect tissues. (a) Cross-section through a muscle cell stretching through the body cavity in the sheep-ked Melophagus ovinus. Note the central arrangement of the nucleus (N) and the lining up of the mitochondria (MI). HE, hemolymph; MF, muscle filament packages (× 13,000). (b) Cross-section through the subesophageal ganglion of a cat flea ( Ctenocephalides felis) showing the axons (A) including mitochondria (MI) and microtubuli (MT) (× 30,000)

Insects, Fig. 6

Diagrammatic representation of the intestinal tract of insects (after Weber) (AN anus, CR crop, ES esophagus, HG hindgut, M mouth, MG midgut, MP Malpighian tubes, PM peritrophic membrane, PR proventriculus, RE rectum, SD salivary duct, SG salivary gland, VC valvula cardiaca, VP valvula pylorica)

Gametogenesis and Fertilization

The testes of insects produce, via a meiosis, numerous filariform haploid  spermatozoa, which are finally endowed with a head containing the nucleus and a midregion including the masses of  mitochondria, and which end in a long motile flagellum extending from the base of the nucleus. During copulation these spermatozoa are transferred directly or via a  spermatophore into the female system. The ovary ( Germarium) of the latter produces mature eggs which in Pterygota contain centrally located  yolk masses (i.e., centrolecithal eggs) and which are surrounded by an eggshell provided with a thin place for fertilization ( Micropyle).

Single mating is the rule in insects, and it involves internal insemination and filling of the  spermatheca. When the mature ovum is shed, the empty ovarial sac contracts, but often remains as a remnant body, thus indicating the individual physiological age (this is helpful, e.g., in populations of Glossina spp.,   Simulium spp., and   Anopheles spp.). When the ova pass the openings of the  spermatheca, fertilization occurs and the final egg shape is regulated by the excretions of the accessory glands. Several spermatozoa enter totally via the  micropyle. During the time needed by the spermatozoa to pass through the yolk, the female nucleus divides meiotically into 4 haploid nuclei. Of these 3 nuclei degenerate, whereas the fourth fuses with the nucleus of the spermatozoon that arrives first. This leads to the diploid set of  chromosomes, the number of which varies among species and even races, e.g., Culex pipiens has 6 chromosomes (= 2 n). This egg starts the embryonation, which in parasitic insects occurs superficially since the cells initially divide as a surface layer on the centrally situated yolk.

Integument

The  cuticle covers the whole body of insects and the anterior and posterior parts of their intestine (Fig. 3). The nonliving cuticular masses are excreted by an epidermis (Hypodermis), which consists of a single layer of cells resting on a basal lamina (basement membrane; Fig. 1). This hypodermis includes a variety of different cells such as the typical epidermal cells, hair-forming cells (tomogen and trichogen cells), oenocytes, sensory cells, and various dermal gland cell types; the latter may form long tubular cytoplasmatic protrusions, which extend close to the surface, thus giving rise to the “cuticular pores” seen in sections (Fig. 1). The “normal” hypodermal cells produce the nonliving cuticle, which is composed of 3 distinct layers, epi-, exo- and endocuticle. The innermost layer is the thick endocuticle which includes  chitin filaments and nontanned protein and thus remains flexible. The exocuticle is also relatively thick and represents the main component of the exoskeleton, since it is built up of chitin and tanned protein (sclerotin). On its outside the exocuticle is covered by the epicuticle, which in general is only 1–3 μm thick; the epicuticle is composed of an inner layer of lipoprotein ( Cuticulin), a polyphenol layer, a wax layer, and finally is covered by a cement layer, thus providing waterproofing and survival in atmospheres that are not water-saturated.

This typical construction of the cuticle, which is regularly molted ( Diptera, Fig. 1), is altered at places where flexibility is needed. Thus, the membranes between sclerites (segments) lack the rigid exocuticle, and the endo- and epicuticle remain smooth to allow body flexion. Molt ( Ecdysis) occurs due to activity of hormones such as ecdyson, neotenin, etc. ( Hormones).

Musculature

The quickly acting muscles of insects as well as arthropods are always of the cross-striated type (Fig. 7a), showing the characteristic arrangement. They are no longer parts of the  body cover, but are segmentally arranged and insert at intersegmental  tendons. The muscular bundles may run in all directions, but in many cases have antagonistic counterparts. Extremities such as the mouthparts, legs, and wings (if present at all) possess muscles that are independent of those of the body, thus allowing countermovements. In addition to these quickly acting striated muscles, the intestine, some other internal organs and external appendages are lined by muscles of the smooth type containing relatively few filaments.
Insects, Fig. 7

Diagrammatic representation of the transmission of  plague bacteria during blood meals of a flea (e.g.,   Xenopsylla cheopis ). (a) The flea sucks bacteria (b) among red blood cells at an infected host. (b) Between 2 blood meals the bacteria reproduce inside the flea and block the opening of its stomach. (c) During the next blood meal the flea regurgitates intestinal contents (with bacteria) into the other host (according to Geigy)

Intestine and Food Uptake

The asymmetrical tube-like gut of adult insects is oriented through the midregion of the body and consists of 3 main portions (Figs. 3 and 8): (1) stomodeum (foregut), (2)  ventriculus (midgut), and (3) proctodeum (hindgut).
Insects, Fig. 8

SEM-micrographs of the mouthparts of dipteran bloodsuckers. Both hide their stylet-like injectors in a protrudible sheath. (a) The tsetse fly   Glossina morsitans . ×50. (b) The  Yellow fever transmitting mosquito Aedes aegypti. The antenna of the females (like in the present picture) possess only a few lateral hairs (×30)

The stomodeum opens anteriorly through the mouth, which is located in the preoral cavity and connected with the excretory ducts of the paired salivary glands. The mouth is armed by (originally) 3 pairs of ventral appendages of the head (caput) modified as mouthparts which are species-specific and adapted to the particular feeding habits. In parasitic insects the following types of mouthparts can be found:
  • The chewing type (e.g., Mallophaga, biting lice) is considered the most basic one, since it is common in many free-living species (beetles, ants, etc.) and consists of large mandibles (to masticate the food), and a pair of separate maxillae I and of fused maxillae II (i.e., labium). Both types of maxillae serve to push the minced food particles into the mouth.

  • The sponging or lapping type is found in most nonbiting dipterans (e.g., Musca, Calliphora; Fig. 2). The mandibles and maxillae are nonfunctional, whereas the remaining parts form a  proboscis with a superficially enlarged tip, the surface of which consists of halfmoon-shaped plates (labella) surrounding the mouth. Foods dissolved by excretion of saliva are ingested in liquid form via the superficial capillary channels, which lead to the mouth.

  • The cutting-sponging (lapping) type is characteristic of tabanids (Fig. 2). Their mandibles are modified as sharp blades and the maxillae appear as long stylets; both may cut the host’s skin. Bloodsucking (lapping) occurs via a sponge-like labium together with the hypo- and epipharynx (i.e., protrusions of the body wall).

  • Piercing-sucking types are present in many bloodsucking ectoparasites such as mosquitoes (Fig. 9), tsetse flies, other flies (Fig. 2), lice ( Lice, Figs. 1, 3), bedbugs ( Bugs, Fig. 2), or fleas (Fig. 3,  Fleas, Fig. 3). Modifications found in the different groups are so great that the homologies between mouthparts can only barely be recognized. In any case, however, 2 different channels are formed by the mouthparts, the larger one is used as a food canal, while the other conveys saliva containing an anticoagulant and several other substances.

Insects, Fig. 9

SEM micrographs of heads of dipteran bloodsuckers. (a, b)   Tabanus sp.; lateral aspect and magnification of the cutting mouth parts (b) (a ×40, b ×60). (c)  Stomoxys calcitrans (×40). (d)   Simulium damnosum, head of a female (×80) (AR arista, AT antenna, CL clypeus, FA compound eye, LA labrum, LB labium, MT maxillar palps, ST piercing apparatus, TI tibia)

The size and shape of the skin-piercing mouthparts are related to the 2 different methods of blood feeding. The mouthparts of vessel or capillary feeders such as some  bugs (Cimex spp., Rhodnius spp.), fleas, and some mosquitoes (e.g., Anopheles spp.) are injected into the lumen of capillaries of suitable caliber, whereas  pool feeders such as most nematocerans (e.g., Simuliidae), some flies (e.g.,   Stomoxys spp., Glossinidae), and tabanids destroy peripheral blood vessels with their armed mouthparts, wait until sufficient blood has collected inside the wound, and then ingest it rapidly, thus visibly distending their stomach.

In parasitic bugs, fleas, lice, tabanids, and some flies (e.g., muscids, tsetse flies), both sexes are hematophagous, whereas in nematocerans such as culicids, simuliids, phlebotomids, and ceratopogonids only the females possess piercing-sucking mouthparts and the males feed exclusively on moisture, etc. or even do not feed.

The mouth opens into the buccal cavity and leads to the pharynx, which eventually acts as a muscular pump and conveys the food down the narrow esophagus to a crop which acts as a storage system. The crop, which in dipterans is a blind-ending diverticulum from the esophagus, in general opens into the narrower valve-like proventriculus, which prevents regurgitation of food from the midgut (cf. Figs. 5 and 8).
  • The ventriculus (midgut, stomach in part) is the main digestive organ and in several parasitic insects is lined with an inner nonadherent tube of chitinous components (peritrophic membrane), which is of great importance in the transmission of pathogens (cf. Fig. 3,  Lice, Fig. 3). The midgut may be divided into different regions with concentrative, digestive, and absorptive functions. In nematocerous larvae and some other insect groups additional gastric ceca may increase the surface area for absorption.

The proctodeum (hindgut) extends posteriorly from the midgut, from which it is separated by a pyloric sphincter. The main function of the hindgut is the resorption of water from the feces and the urine. The components of the latter come from the Malpighian tubules, which open at the border between the mid- and hindgut. The hindgut usually has an intestinal part, followed by the colon and rectum, and finally opens into the ventrally located anus (Fig. 8).

Excretory System

In insects the Malpighian tubules are of ectodermal origin and function as the main excretory system; they are blind-ending, tube-like appendages of the intestine and open at the border between the mid- and hindgut (Fig. 8). Apparently the waste-containing hemolymph circulates in the hemocoel near these structures, the number of which is species-specific. The main function is the absorption of uric acid (as sodium and potassium salts) and their discharge into the lumen of the intestine, from where the excretory products are passed with the feces. Strict water resorption usually occurs at the base of the Malpighian tubules and in the rectum, avoiding the waste of water.

Nervous System

The nervous system of insects mainly consists of a rather thick rope ladder-like ventral chord with a pair of ganglia within each segment. In several insect groups this rope-ladder becomes, however, condensed to a rather thick chord. The “brain” (i.e., cerebral ganglion) is comprised of the enlarged supra = epiesophageal ganglion (Figs. 5 and 7b), which is connected by connectives with the subesophageal ganglion, which has been formed due to the fusion of the ganglia of the mouthparts. The epiesophageal ganglion is subdivided into 3 regions:
  • The protocerebrum is rather large; it innervates the compound eyes with large lobi optici and functions as center of associations.

  • The smaller deutocerebrum innervates the antennae, which are equipped with numerous sensillae.

  • The tritocerebrum forms a commissure running below the intestine.

The rather large ganglia pairs of the pro-, meso-, and metasomal segments innervate the 3 pairs of legs and – if present – the muscles of the wings. In the abdomen 7 pairs of ganglia are present. Ring nerves to steer the different body organs initiate from these sites.

The additional visceral (i.e., vegetative) nerve system consists of 3 regions:
  • The stomatogastric part, which innervates the mouth and anterior intestine comprising frontal, hypocerebral and ventricular ganglia, the  corpora cardiaca, and the  corpora allata.

  • A singular ventral nerve chord which innervates the stigmata.

  • The caudal system which is responsible for the intestine and the gonads.

The whole nervous system of insects is 5,000 times more sensible to insecticides of the recently used pyrethrum/pyrethroid family than that of vertebrates – a fact which is used to control pests and bloodsucking insects (Ectoparasiticides,  Arthropodicidal Drugs).

Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Fakultät für Biologie, AG Zoologie/ParasitologieRuhr-Universität BochumBochumGermany
  2. 2.Institut für Zoomorphologie, Zellbiologie und ParasitologieHeinrich-Heine-UniversitätDüsseldorfGermany