Entomology 3rd edition - C.Gillott - Chapter 17 ppsx

Ultrastructural examination of the heart muscle cells reveals, however, that they contain, in addition to contractile elements, prominent Golgi complexes and vesicles, suggesting that th

of both blood and lymph of vertebrates Thus, the fluid fraction (plasma) is important inproviding the correct milieu for body cells, is the transport system for nutrients, hormones,and metabolic wastes, and contains elements of the immune system, while the cellular com-ponents (hemocytes) provide the defense mechanism against foreign organisms that enterthe body and are important in wound repair and the metabolism of specific compounds.

2 Structure

The primary pump for moving hemolymph around the body is a middorsal vessel thatruns more or less the entire length of the body (Figure 17.1) The posterior portion of thevessel has ostia (valves) and is sometimes known as the heart, whereas the cephalothoracicportion, which is often a simple tube, may be termed the aorta (Figure 17.1A) In someinsects the heart is the only part that contracts, but in many others the entire vessel iscontractile The vessel is held in position by connective tissue strands attached to the dorsalintegument, tracheae, gut, and other organs and by a series of paired, usually fan-shaped,alary muscles Normally, the vessel is a straight tube, though in many species the aorta mayloop vertically Anteriorly the aorta runs ventrally to pass between the corpora cardiacaand under the brain Generally the dorsal vessel is closed posteriorly; however, in Diplura,

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FIGURE 17.1. (A) Ventral dissection of the field cricket, Acheta assimilis, to show dorsal vessel and associated structures; and (B) circulatory system of Campodea augens (Diplura) showing anterior and posterior arteries

running off the dorsal vessel [A, after W L Nutting, 1951, A comparative and anatomical study of the heart and

accessory structures of the orthopteroid insects, J Morphol 89:501–597 By permission of Wistar Press B, from

a figure kindly supplied by Dr G¨unther Pass.]

Archaeognatha, Zygentoma, and some Ephemeroptera the dorsal vessel connects at its rearwith arteries that run along the cerci and median caudal filament (Gereben-Krenn and Pass,2000) In Diplura an artery also supplies each antenna (Figure 17.1B), and in Dictyopteraand some Orthoptera there are pairs of segmental arteries in the abdomen (Hertel and Pass,2002) However, except as noted, in pterygotes circulation to appendages is achieved bymeans of accessory pulsatile organs and septa (see below)

In most insects the dorsal vessel is well tracheated The heart may not be innervated ormay receive paired lateral nerves from the brain and/or segmental ventral ganglia Ostia may

be simple, slitlike valves or deep, funnel-shaped structures in the wall of the heart, or internal

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SYSTEM

FIGURE 17.2. Incurrent ostia of Bombyx

shown during diastole and systole

Ar-rows indicate direction of hemolymph flow.

[After R F Chapman, 1971, The Insects:

Structure and Function By permission

of Elsevier/North-Holland, Inc., and the

author.]

flaps (Figure 17.2) Their position and number are equally varied They may be lateral,

dorsal, or ventral and may be as numerous as 12 pairs (in cockroaches) or as few as 1 pair

(in some dragonflies) Ostia are usually incurrent, that is, they open to allow hemolymph

to enter the heart but close to prevent backflow In some orthopteroid insects, however,

some ostia are excurrent Histologically, the dorsal vessel in its simplest form comprises a

single layer of circular muscle fibers, though more often longitudinal and oblique muscle

layers also occur Ultrastructural examination of the heart muscle cells reveals, however, that

they contain, in addition to contractile elements, prominent Golgi complexes and vesicles,

suggesting that the insect heart is secretory and, like that of vertebrates, may have a more

significant role in homeostasis than just pumping hemolymph (Locke, 1989)

Assisting in directing the flow of hemolymph, especially in postlarval stages, are

var-ious diaphragms (septa) (Figure 17.3) that include both connective tissue and muscular

elements The spaces delimited by the diaphragms are known as sinuses The pericardial

septum (dorsal diaphragm) lies immediately beneath the dorsal vessel and spreads between

the alary muscles Laterally, it is attached at intervals to the terga and in most species has

FIGURE 17.3. Diagrammatic transverse section through abdomen to show arrangement of septa [From

R E Snodgrass, Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc Used with permission

of McGraw-Hill Book Company.]

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openings so that the pericardial sinus is in effect continuous with the perivisceral sinus.Ventrally, a perineural septum (ventral diaphragm) may occur, which cuts off the perineu-ral sinus from the perivisceral sinus Generally, the ventral diaphragm is restricted to theabdomen and occurs only in species whose ventral nerve cord extends into this region ofthe body (Miller, 1985) It is capable of performing posteriorly directed undulations andmay have openings It may receive motor nerves from segmental ganglia, which regulatethe rate at which it undulates, though the undulations originate myogenically In some in-sects, for example, caddisflies and cockroaches, the ventral diaphragm is reduced to a fewtransverse or longitudinal muscles, respectively Frequently, there is a close physical asso-ciation between the diaphragm (or its vestiges) and the nerve cord Thus, in cockroaches

and Pseudaletia unipuncta the actions of the longitudinal muscle remnants cause the nerve

cord to oscillate laterally, bringing it into greater mix with the hemolymph and possibly

improving hemolymph flow (Koladich et al., 2002) Hemolymph circulation through the

legs and palps of some insects is assisted by the presence of a longitudinal septum thatpartitions the appendage into afferent and efferent sinuses

To further facilitate hemolymph flow, especially through appendages, accessory satile organs (auxiliary hearts) commonly occur (Pass, 1998, 2000) These have been iden-tified in the head, antennae, thorax, legs, wings, and ovipositor In many species they aresaclike structures that have a posterior incurrent ostium and an anteriorly extended vessel

pul-In antennal pulsatile organs the vessel may run the length of the appendage but is perforated

at intervals to permit exit of hemolymph The wall of the sac may be muscular, so thatconstriction of the sac is the active phase, and dilation results from elasticity of the wall,

or the sac may have attached to it a discrete dilator muscle, and constriction is due to thesac’s elasticity In some situations, for example, the legs of Orthoptera and Hemiptera, theaccessory pulsatile organ is simply one or two small muscles that attach to the longitudinalseptum Indeed, in Hemiptera, the organ is clearly derived from a skeletal muscle, the pre-tarsal depressor (Figure 14.5C) (Hantschk, 1991) Contraction narrows the efferent sinus,while enlarging the afferent sinus Valves ensure that hemolymph is pushed toward the limbtip, then back toward the body cavity Normally, accessory hearts are quite separate from thedorsal vessel, though in some Odonata they are connected via short vessels with the aortainto which they pump hemolymph Most accessory pulsatile organs are not innervated.Hemopoietic organs have been described for a number of insects For example, in

Gryllus there are pairs of such organs, in the second and third abdominal segments, directly

connected with the dorsal vessel Like those of vertebrates, the hemopoietic organs serveboth as the site of production of at least some types of hemocytes and as centers forphagocytosis The same cells within the hemopoietic organ can carry out both of thesefunctions, though not simultaneously; thus, during periods of infection, division of the cells

to form new prohemocytes is greatly retarded

At specific locations in the circulatory system are sessile cells, usually conspicuouslypigmented, called athrocytes (Locke and Russell, 1998) They occur singly, in small groups,

or form distinct lobes, and are always surrounded by a basal lamina, a feature that guishes them from hemocytes In most species athrocytes are situated on the surface ofthe heart (occasionally also along the aorta), and these are referred to as pericardial cells

distin-They may also be found as scattered cells in the fat body (in Lepisma), in clusters at the bases of legs (in Gryllus and Periplaneta), or as a garland of cells around the esophagus (in

some larval Diptera) When mature they may contain several nuclei, as well as dria, Golgi apparatus, and pigment granules or crystals of various colors The cells are able

mitochon-to accumulate colloidal particles, for example, certain dyes, hemoglobin, and chlorophyll,

THE CIRCULATORY

SYSTEM

which led to an early suggestion that they segregated and stored waste products (hence their

alternate name of nephrocytes) The usual view is that the cells accumulate and degrade

large molecules such as proteins, peptides, and pigments, and the products are then used or

excreted However, their structure includes much rough endoplasmic reticulum and

well-developed Golgi complexes, characteristics of cells producing protein for export Indeed,

Fife et al (1987) demonstrated that the pericardial cells synthesized and secreted several

proteins into the hemolymph

3 Physiology

3.1 Circulation

Contractions of the dorsal vessel and accessory pulsatile organs, along with movements

of other internal organs and abdominal ventilatory movements (coelopulses), serve to move

hemolymph around the body (Miller, 1997) In Periplaneta larvae, for example, circulation

time is 3–6 minutes; in Tenebrio the time for complete mixing of injected radioisotope is

8–10 minutes Generally hemolymph is pumped rapidly through the dorsal vessel but moves

slowly and discontinuously through sinuses and appendages

The direction of hemolymph flow in most insects is indicated in Figure 17.4A–C

Hemolymph is pumped anteriorly through the dorsal vessel from which it exits via either

excurrent ostia of the heart or mainly the anterior opening of the aorta in the head The

resultant pressure in the head region forces hemolymph posteriorly through the perivisceral

FIGURE 17.4. Diagrams showing direction of hemolymph flow (A) Longitudinal section; (B) transverse section

through thorax; and (C) transverse section through abdomen Arrows indicate direction of flow [After V B.

Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co By permission of the author.]

CHAPTER 17

and perineural sinuses Undulations of the ventral diaphragm aid the backward flow ofhemolymph Relaxation of the heart muscle results in an increase in heart volume, and, bynegative pressure, hemolymph is sucked in via incurrent ostia As noted earlier, circulationthrough appendages is aided by accessory pulsatile organs In most insects hemolymphenters the wings via the anterior veins and returns to the thorax via the anal veins Thoughthe structure of wing pulsatile organs is varied, they always operate by sucking hemolymphout of the posterior wing veins (Pass, 1998, 2000) In some Coleoptera and Lepidoptera,tidal flow of hemolymph occurs in the wings; that is, hemolymph flows into or out of allveins simultaneously

In apterygotes and mayflies hemolymph flow is bidirectional (Figure 17.1B) Anterior

to a valve located in the heart at about the level of the eighth abdominal segment, hemolymphflows forward toward the head, while behind the valve the hemolymph is pushed backwardalong arteries that terminate at the tips of the cerci and median filament (Gereben-Krennand Pass, 2000) Reversal of heartbeat may also occur and is characteristically seen in pupaeand adults of Lepidoptera and Diptera

In some actively flying insects, for example, locusts, butterflies, saturniid moths, andpossibly some Hymenoptera, as well as in diapausing lepidopteran pupae, hemolymphmovements are closely coordinated with the ventilation movements for gas exchange(Chapter 15, Sections 3.2 and 3.3) Abdominal pumping not only improves gas exchangewithin the tracheal system but also brings about tidal flow (oscillating circulation) ofhemolymph In other words, hemolymph flows back and forth between the abdomen andthe anterior part of the body Tidal flow of hemolymph into the abdomen is aided by reverseperistalsis of the dorsal vessel (Miller, 1997)

Control of circulation is especially important in large flying insects such as bumble bees,dragonflies, and night-flying moths that thermoregulate Thermoregulation allows theseinsects to warm up their wing musculature at low ambient temperatures and to dissipateheat produced during flight at high temperatures At low ambient temperatures, the heartbeat

is weak and contraction of the ventral diaphragm infrequent, so that heat produced by flight contraction of wing muscles is retained within the thorax As the thoracic temperaturebecomes suitable for flight, heartbeat rate and amplitude increase, as do the frequency andstrength of contractions of the ventral diaphragm, taking heat away from the thorax toprevent overheating (Miller, 1997)

pre-3.2 Heartbeat

Contraction of the heart (systole) is followed, as in other animals, by a phase of ation (diastole) during which muscle cell membranes become repolarized A third phase,diastasis, may follow diastole, when the diameter of the dorsal vessel suddenly enlargesbecause of the influx of hemolymph Diastole in many insects seems to be passive, that is,the result of natural elasticity of the heart muscle Though alary muscles may be quite welldeveloped in such species, they apparently have no role in the relaxation process They havebeen shown to be electrically inexcitable in locusts and cockroaches, and cutting them has

relax-no effect on the rate and strength of the heartbeat In a few species structural integrity ofthe heart and alary muscles is vital, and cutting the alary muscles terminates the heartbeat

In most pterygotes, where hemolymph flow is unidirectional, contraction of the dorsalvessel begins at the posterior end and passes forward as a peristaltic wave Experimen-tally contraction can be induced at any point along the length of the vessel and individualsemiisolated segments (portions of the heart with tergum still attached) continue to beat

THE CIRCULATORY

SYSTEM

rhythmically These observations suggest that the heartbeat is normally coordinated by a

pacemaker located posteriorly In adult Manduca sexta in which heartbeat reversal occurs,

distinct pacemakers exist for the anteriorly and posteriorly directed contractions (Dulcis

et al., 2001).

Whether or not an insect heart is innervated, its beat is myogenic, that is, the beat

originates in the heart muscle itself (Jones, 1977; Miller, 1985, 1997) This contrasts with

the situation in Crustacea and Arachnida, which have neurogenic hearts For innervated

insect hearts, it is generally assumed that, as in vertebrates, control of the rate and amplitude

of the heartbeat resides in the cardiac neurons However, as Miller (1985) pointed out, such

regulation has been demonstrated in only a few cases

The rate at which the heart beats varies widely both among species and even within an

individual under different conditions In the pupa of Anagasta kuhniella ¨ , for example, the

heart beats 6–11 times per minute In larval Blattella germanica rates of 180–310 beats/min

have been recorded (Jones, 1974) Many factors affect the rate of heartbeat Generally, there

is a decline in heartbeat rate in successive juvenile stages, and in the pupal stage the heart

beats slowly or even ceases to beat for long periods In adults the heart beats at about the

rate observed in the final larval stage Heartbeat rate increases with activity, during feeding,

with increase in temperature or in the presence of carbon dioxide in low concentration, but

is depressed in starved or asphyxiated insects Hormones, too, may affect heartbeat rate

Authors have reported a wide range of cardioaccelerating and cardioinhibiting factors,

in-cluding juvenile hormone, neurosecretory peptides, octopamine, and 5-hydroxytryptamine

However, in many instances, an effect of these substances on the metabolism of the insect

may cause the change in heartbeat rate observed

As noted, the ventral diaphragm and accessory pulsatile organs may or may not be

innervated Thus, it may be anticipated that, as with the dorsal vessel, contraction of these

structures may be controlled neurally and hormonally or by hormones alone

Pharmacolog-ical studies have shown that a range of amines and small peptides can modulate contraction

of these structures However, immunohistochemistry has identified amine- and

peptide-releasing neurons terminating at these structures, tending to cloud the picture with respect

to which system is regulating their activity (Hertel and Pass, 2002; Koladich et al., 2002).

4 Hemolymph

Hemolymph, like the blood of vertebrates, includes a cellular fraction, the hemocytes,

and a liquid component, the plasma, whose functions are broadly comparable with those

found in vertebrates Several of the features of hemolymph, however, contrast markedly

with what is seen in vertebrates First, associated with the evolution of a tracheal system,

hemolymph has no gas-transporting function, except perhaps in some chironomid larvae In

addition, the composition of hemolymph (especially in the more advanced endopterygotes)

is both very different from that of blood and is much more variable on a day-to-day basis

Among the trends seen in the evolution of the higher endopterygotes are substitution of

organic molecules for the predominant inorganic ions (sodium and chloride), an increase in

the proportion of divalent to monovalent cations, and an increase in the importance

(quan-titatively) of organic phosphate Further, as noted in the previous chapter, monosaccharide

sugars are generally of little importance in the hemolymph and are replaced by the

disac-charide trehalose The use of this molecule rather than the reducing sugar glucose appears

to be an adaptation to overcome problems of osmotic pressure and chemical reactivity that

CHAPTER 17

would result if the monosaccharide were the major form of fuel in the hemolymph (Wheeler,1989) In many insects the hemolymph osmotic pressure is held reasonably constant over arange of environmental conditions In other species, the osmotic pressure changes in paral-lel with the environmental conditions, yet the body cells are able to tolerate these changes(Chapter 18, Section 4)

4.1 Plasma

4.1.1 Composition

Plasma contains a large variety of components both organic and inorganic whose relativeproportions may differ greatly both among species and within an individual under differentphysiological conditions Despite this variability, some general statements may be made

In primitive orders, the predominant cation is sodium, with potassium, calcium, andmagnesium present in low proportions The major anion is chloride, though plasma alsocontains small amounts of phosphate and bicarbonate These inorganic constituents are themajor contributors to the hemolymph osmotic pressure (Figure 17.5A,B)

In higher orders certain trends can be observed The relative importance of sodiumdecreases at the expense of potassium and, especially, magnesium Chloride also decreases

in importance and is replaced by organic anions, especially amino and carboxylic acids nally, the relative contribution that inorganic ions make to the hemolymph osmotic pressuredeclines, and organic constituents become the major osmotic effectors (Figure 17.5D,E).Superimposed on these phylogenetic relationships may be dietary and ontogeneticconsiderations, especially with respect to the cationic components of hemolymph Thus,zoophagous species generally have a larger proportion of sodium in the hemolymph, in con-trast to phytophagous species where magnesium (derived from chlorophyll) and potassiumare the major cations (Figure 17.5C) The ionic composition may also change with stage

Fi-of development, in endopterygotes at least, though whether this is related to a change Fi-ofdiet which, of course, may also occur from the juvenile to the adult stage, does not seem

to have been considered For example, in the exopterygotes Aeshna cyanea (Odonata),

FIGURE 17.5. Relative contributions to osmotic pressure of the components of hemolymph in different insect groups Each column represents 50% of the total osmolar concentration [After D W Sutcliffe, 1963, The chemical

composition of hemolymph in insects and some other arthropods in relation to their phylogeny, Comp Physiol.

Biochem 9:121–135 By permission of Pergamon Press, Elmsford, NY.]

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SYSTEM

Periplaneta americana (Dictyoptera), and Locusta migratoria (Orthoptera) and the

en-dopterygote Dytiscus (Coleoptera), the composition of the hemolymph is similar in larvae

and adults, but so, too, is the diet In contrast, in a few endopterygote species, specifically

Lepidoptera and Hymenoptera, for which data are available, larval hemolymph is of the

high magnesium type, whereas adult hemolymph has a much greater sodium content Again,

however, the fact that the diet of the adult (if it feeds) is typically different from that of the

larva apparently has not received consideration

As Florkin and Jeuniaux (1974) pointed out, insects whose hemolymph contains such

large quantities of magnesium and potassium must have become adapted so that

physiologi-cal processes, especially neuromuscular function, can be carried out normally because these

ions are detrimental Hoyle (1954, cited in Florkin and Jeuniaux, 1974) suggested that the

high magnesium-potassium type of hemolymph characteristic of phytophagous

endoptery-gote larvae might reduce, through an effect on the nervous system, locomotor activity of

larvae, so that they would tend to remain close to their food The adult, in contrast, is usually

much more active and possesses the more primitive high sodium type of hemolymph

Organic acids are important hemolymph constituents, especially in juvenile

endoptery-gotes Carboxylic acids (citric,α-ketoglutaric, malic, fumaric, succinic, and oxaloacetic),

which in the hemolymph are anionic, are present in large amounts and may neutralize

almost 50% of the inorganic cations They are apparently synthesized by insects (or by

symbiotic bacteria), as their levels in the hemolymph are independent of diet Whether

these acids, which are components of the Krebs cycle, also have a metabolic function in the

hemolymph is not known Insects, especially endopterygotes, also characteristically have

high concentrations of amino acids in their hemolymph The proportions of amino acids

vary among species and within an individual according to diet and developmental and

phys-iological state, though glutamine, glycine, histidine, lysine, proline, and valine generally

each constitute at least 10% of the total amino acid pool The amino acids make a significant

contribution to the hemolymph osmotic pressure, though whether they function as cations

or as anions depends both on the pH of the fluid (usually between 6.0 and 7.0) and on the

individual amino acid In addition, some have important metabolic roles

The hemolymph protein concentration is generally about 1% to 5% but varies with

species and individual physiological states For example, it is low in starved insects and high

in females with developing oocytes In endopterygotes the protein concentration often

in-creases through larval life, especially in the final instar, but then declines during pupation As

noted in Chapter 16 (Section 5.4), the fat body is the major source of the many proteins found

in the hemolymph though other tissues, notably midgut epithelium, epidermis, pericardial

cells, and hemocytes, also contribute For relatively few of these proteins has the function

been determined Some, for example, lysozyme, phenoloxidase, cecropins, and attacins,

are important in prevention of infection The female-specific proteins (vitellogenins) are

selectively accumulated by oocytes during yolk formation Some proteins act as transport

agents for lipids and juvenile hormone Yet others may serve simply as concentrated stores

of nitrogen that can be degraded for use in growth and metabolism Among the enzymes

identified in hemolymph are hydrolases (e.g., amylase, esterases, proteases, and trehalase),

dehydrogenases and oxidases important in carbohydrate metabolism, and tyrosinase

The principal carbohydrate in the hemolymph of most insects is the disaccharide

tre-halose, which serves as a source of readily available energy (Chapter 16, Section 5.2)

Monosaccharides and polysaccharides normally occur in only small amounts Glycerol and

sorbitol are sometimes present in high concentration in the hemolymph of overwintering

stages where they serve as antifreezes

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Free lipids seldom occur in high concentrations in hemolymph, except in some speciesafter feeding, during flight, or at metamorphosis when they are being transported to sites ofuse or storage

Other constituents of hemolymph include phosphate esters, urates, and traces of othernitrogenous waste products, amino sugars such as acetylglucosamine produced during di-gestion of the cuticle, pigments (often conjugated with protein), and hormones, which may

be transported to their sites of action in combination with protein

4.1.2 Functions

Apart from the specific functions of particular components, which were outlined in theabove consideration of its composition, the plasma has some important general functions Itserves as the medium in which nutrients, hormones, and waste materials can be transported

to sites of use, action, and disposal, respectively It is an important site for the storage,usually temporary, of metabolites Plasma is the source of cell water, and during periods ofdesiccation its volume may decline at the expense of water entering the tissues By virtue

of some components (proteins, amino acids, carboxylic acids, bicarbonate, and phosphate)

it is a strong buffer and resists changes in pH that might occur as a result of metabolism As

a liquid, it is also used to transmit pressure changes from one part of the body to another It

is used hydrostatically, for example, to maintain the turgor necessary for movement in bodied insects, split the old exocuticle during ecdysis, expand appendages after ecdysis,evert structures such as the penis, and extend the labium of larval Odonata The plasmaalso has an important thermoregulatory function in many actively flying insects Variousstructural and physiological features of the circulatory system have evolved that allow heat to

soft-be retained in the thoracic region (at lower ambient temperatures) or to soft-be easily transported

to the abdomen and hence dissipated when ambient temperatures are high (Miller, 1985)

4.2 Hemocytes

4.2.1 Origin, Number, and Form

The embryonic origin of hemocytes is considered in Chapter 20, Section 7.5 In bryonic stages of many species, hemocytes are produced in discrete hemopoietic organs; in

postem-other species multiplication of hemocytes takes place in the hemocoel per se, either as the

cells circulate or as they rest on the surface of tissues

In many species all or nearly all of the hemocytes are in circulation; in some species veryfew hemocytes circulate, the great majority remaining loosely attached to tissue surfaces

In adult mosquitoes there are no circulating hemocytes (Jones, 1977) Hemocyte countsare normally highly variable within the same species, as well as differing among species

Thus, in Periplaneta americana, counts ranging from 45,000–120,000 hemocytes/µl may

be measured As the insect has about 170µl of hemolymph, it will have between 7 and

20 million circulating cells

According to Crossley (1975), there is evidence that the number of circulating cytes may depend on the hormone titer of the hemolymph Both ecdysone and juvenilehormone are said to stimulate an increase in the number of circulating hemocytes, es-pecially plasmatocytes In some instances, the increase seems to be related to increasedmobility of preexisting cells rather than an increased rate of cell multiplication, though howhormones bring about this effect is not known

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FIGURE 17.6. Different types of hemocytes [After R F Chapman, 1971, The Insects: Structure and Function.

By permission of Elsevier/North-Holland, Inc., and the author.]

Though several types of hemocytes have been recognized, which differ in size,

stain-ability, function, and cytology (including fine structure) (Figure 17.6), their classification

and relationships have proven difficult This difficulty stems partly from the natural

struc-tural variability and multifunctional nature of some hemocytes both within and among

species, and partly from the differences in methodology and criteria used to distinguish

different hemocytes (Arnold, 1974; Crossley, 1975; Jones, 1977) Notwithstanding these

difficulties, it is apparent that three types of hemocytes are common to almost all insects,

though one or more additional types may also occur in a given species In this account, the

scheme of Arnold (1974) is followed

The three types common to most insects are prohemocytes, plasmatocytes, and granular

hemocytes (granulocytes) Prohemocytes (stem cells) are small (10µm orµµ less in

diame-ter), spherical, or ellipsoidal cells whose nucleus fills almost the entire cytoplasm They

are frequently seen undergoing mitosis and are assumed to be the primary source of new

hemocytes and the type from which other forms differentiate Plasmatocytes (phagocytes)

are cells of variable shape and size, with a centrally placed, spherical nucleus surrounded

by well vacuolated cytoplasm In the cytoplasm are a well-developed Golgi complex and

endoplasmic reticulum, as well as many lysosomes The cells are capable of amoeboid

movement and are phagocytic Granulocytes are usually round or disc-shaped, with a

rel-atively small nucleus surrounded by cytoplasm filled with prominent granules In some

species they are amoeboid and phagocytic which, together with the occurrence of

interme-diate forms, suggests that they may be derived from plasmatocytes More often, they are

non-motile and appear to be involved in intermediary metabolism

Other types of hemocytes include adipohemocytes, oenocytoids, spherule cells, and

cystocytes As their name indicates, the adipohemocytes are cells whose cytoplasm normally

contains droplets of lipid In addition to lipid droplets, the cytoplasm may have non-lipid