The project describes the morphology of a drosophilid, Drosophila sp., collected from the Rajshahi City Corporation area. The taxonomic characters of the fly are also evaluated.
Drosophila melanogaster is a fruit fly, a little insect about 3mm long, of the kind that accumulates around spoiled fruit. It is also one of the most valuable of organisms in biological research, particularly in genetics and developmental biology.
Drosophila has been used as a model organism for research for almost a century, and today, several thousand scientists are working on many different aspects of the fruit fly. Its importance for human health was recognised by the award of the Nobel Prize in medicine/ physiology to Ed Lewis, Christiane Nusslein-Volhard and Eric Wieschaus in 1995.
Over the course of the past 25 years, the tools of phylogenetic systematics have been applied to test existing taxonomies and to propose novel relationships among taxa. Phylogenetic systematics seeks to use explicit character information (e.g., morphology, DNA sequence) to infer evolutionary relationships among organisms. This approach is an improvement over traditional taxonomy because of the explicit, repeatable analytical methods used to infer evolutionary relationships.
Powerful statistical methods can be applied to place taxonomy in an hypothesis-testing framework, an important consideration when making statements about morphological, developmental and ecological innovation, character evolution and phylogenetic relationships.
A given group of organisms can be classified as monophyletic, paraphyletic or polyphyletic (Fig. 1A–C). Monophyletic groups, or clades, consist of a common ancestor and all descendants of that ancestor (Fig. 1A). Basing taxonomic structure on clades is a powerful approach because it provides information about the composition and exclusivity of a group. Shared derived characters that delimit a group can also be used to tentatively place newly discovered species.
Paraphyletic groups contain an ancestor and only some descendants of that ancestor (Fig. 1B). Because some descendants of an ancestor are not present in a paraphyletic group, these are less useful when trying to make an explicit statement about the evolutionary history of a lineage. Polyphyly is similar to paraphyly in that not all ancestors or descendants are members of the same group. Polyphyletic groups originate from multiple points on a phylogenetic tree (Fig. 1C).
Phylogenetics and Taxonomy of Drosophila
The debate between those who prefer to divide large genera into smaller units and others who propose to lump multiple genera into a single taxon is probably the oldest in taxonomy. The genus Drosophila provides an interesting modern example. This genus is polyphyletic and thus one will need to either divide it into smaller, monophyletic units or propose a single large clade that encompasses all currently described Drosophila species (Fig. 2A–C). Figure 2A shows a scenario where the genus Drosophila has been modified to include a number of other genera in order to create a large monophyletic group containing over 2,000 species. Under such a scheme, several genera (e.g., Scaptomyza, Hirtodrosophila, Zaprionus) are included as subgenera of Drosophila and their generic names must change. This action would create homonyms, species names that are identical yet refer to different species. For example, D. heedi and Scaptomyza heedi are two separate taxonomic entities.
However, if these two genera are at one point considered synonyms, one of those species names must change to avoid confusion about which morphological species the name D. heedi is referring to. Creating a larger genus Drosophila would generate over 100 homonyms.
Figure 2B illustrates two scenarios where the genus Drosophila is split into smaller monophyletic groups. While these would require some change in generic names, there would be no issue with homonyms. However, the first scenario would require a change in some generic names. This is due to the rules of nomenclature and how types are designated. When new species are described, a single specimen is designated as the holotype, the specimen to which all others are compared when determining species identity.
The rule of priority states that the earliest described species in a genus is designated the type of that genus. This provides a link between the name and the morphological definition of a species and/ or genus. All newly described species are placed into genera based on their similarity to the type species of that genus. D. funebris Fallen 1823 is the type of the genus Drosophila. The rule of priority also holds for genera.
The earliest valid genus name should be used whenever possible. When genera are synonymized, or equated with one another, the oldest type species determines which genus name is used. All subsequent generic names are considered junior synonyms and are not used, although they may be resurrected in the future if the taxonomy changes. D. funebris, the type of the genus Drosophila, is not in the same clade as D. melanogaster.
Thus, if the genus Drosophila were to be split into smaller monophyletic groups and D. funebris maintained as the type of the genus, according to the rules of nomenclature the name Drosophila melanogaster would change to Sophophora melanogaster (Fig. 2B). An alternate approach to splitting the genus Drosophila is also shown in Figure 2B. This involves proposing to redesignate D. melanogaster as the type of the genus Drosophila. Such an action, however, requires making an exception to the rules of nomenclature and an application to do this has been made to the International Committee of Zoological Nomenclature.
4-10 Most importantly, the rules of nomenclature clearly state that the Commission may only render decisions on issues of nomenclature and should not endorse any single classification scheme—taxonomic, phylogenetic or otherwise. 11 By placing this issue before the Commission, the petitioners are asking for an endorsement of their taxonomic scheme (Fig. 2B, splitting the genus) over alternate scenarios (Fig. 2A and C) that are equally well supported by the data and involve less drastic taxonomic change. If the Commission votes in favor of the van der Linde et al. proposal, they would make D. melanogaster the type of the genus Drosophila and preserve the binomen, Drosophila melanogaster. Should the Commission ultimately oppose the petition, there is, in fact, still no change to the name D. melanogaster.
Description of Dipteran insects:
Insects with a single pair of membranous wings, the hind pair modified into halteres. Mouthparts suctorial, usually forming a proboscis and sometimes adapted for piercing: mandibles rarely present: labium usually distally expanded into a pair of felshy lobes. Prothroax small and fused with the large mesothorax: tarsi commonly 5 segmented. Metamorphosis complete, larvae eruciform and apodous, frequently with the head reduced and retracted: tracheal system variable, most often amphipneustic, pupa either free or enclosed in the hardened larval cuticle or puparium: wing- tracheation reduced.
The Diptera are one of the largest orders of insects including over 64,000 described species and approximately 5,200 species are known from the British Isles. Structurally Diptera are among the most highly specialized members of their class. The imagines of almost all the species are diurnal and the majorities are either flower- lovers, which feed upon nectar etc or frequent decaying organic matter of various kinds. Although these two habits predominate, a considerable number of flies are predacious and live on various insects.
Adult Diptera (true flies) are easily recognized by their single pair of wings. By this criterion they can be distinguished from nearly all other winged insects. The hind pair of wings is replaced by a pair of drumstick- like structures called halteres.
In both the number of individuals and the number of species the Diptera group is a large one.
The characteristics of the family Drosophilidae are briefly as following:
They are small flies, attracted to fermenting substances; they have a very characteristic slow, ponderous, hovering flight. In colour they range from yellow, through brown to black; typically they have bright red eyes, they have no hairs or bristles on the mesopleuron. In the wing the costal vein is broken twice, once near the humeral cross vein and again close to the top of the vein. First vein is short and the anal cell and vein sixth is always present. There is no trace of a cleft on the outer (lateral) surface of the second segment of the antenna. The third segment of antenna is rounded and short.
The arista is generally plumose (much branched), with a fork at he end called the apical fork, formed from the main stem and the last branch. On the head, the postvertical bristles are usually convergent (pointing towards each other) but the 2 or 3 orbital bristles always curve down or up, never inwards and the middle (shortest) orbital bristle is always set somewhat closer to the eye than are the others.
Within the family Drosophilidae the genus Drosophila can be distinguished by the following additional characters: the eyes are covered with a dense short pile of hairs, the postvertical bristles are well developed and usually cross over each other, and there are three orbital bristles. On the thorax there are two pairs of drosocentral bristles. The majority of Drosophila species have six or eight rows of acrostichal bristles which lie just in front of he anterior pair of dorsocentral bristles.
Within the genus Drosophila at least eleven smaller divisions or sub genera are now recognized and within certain of these the individual species are arranged in groups.
Small flies occurring about flowing sap, decaying fruits, wine, vat, vinegar, fermentation products, with red eyes. Antennae with 2nd segment cleft, arista dorsally pectinate with superior orbital bristles, costa broken near sub costa. Larva – predatory on fruit. It segmented body and each segment surrounded by a girdle of minute hook like spines. Anal segments of larva with three pairs of lobes and a respiratory retractile process. Larvae feed on decaying fruits, fungi; some are leafminers, while others are predators on alergodids and coccids.
Apart from the males of the Coccidae or scale- insects, the Diptera stand alone amongst the orders of Insects in the peculiarity of possessing only two wings, being represented by a small organ on each side of the thorax behind and below the wing- root, known as the winged forms of the Order and generally present also in the very few wingless ones. In rare cases the female only is wingless.
The mouth parts exhibit a wide diversity in structure, but they are nearly always in the shape of a proboscis, formed for sucking, or in some groups, for piercing the skin of animals or man for the purpose of imbibing the blood.
The antennae also show extreme range in form but are valuable as one of the primary means of classification. The palpi have from one to five joints, the usual number in the Nematocera being four; in the Brachycera and the Muscide two of which the first is small and often more of less rudimentary.
The wing exhibit a multiplicy of schemes of venation all referable ultimately to a common general plan and this latter character the venation perhaps affords ht soundest basis for classifying the families at any rae so far as the perfect insects only are concerned.
External anatomy of a Fly.
The upper part of the head lying between the eyes is known as the vertex and an imaginary line dividing it from the back of the head or occiput, is called the vertical margin. Some times a difference of colour clearly defines the limits of these areas, but more frequently such is not the case.
The frons or front is the whole space between the eyes from the vertical margin to an imaginary transverse line above the base of the antennae. The vertex is really therefore simply upper part of the fronts, but it is often occupied by a somewhat conspicuous tubercle, or by a more or less triangle, the apex of he latter pointing downwards, that is towards the antennae. These ocelli are in many families absent in other reduced to two in number and they may in a few groups be placed more or less in a straight line across the upper part of the fronts.
Those Diptera in the males of which the eyes touch each other in front for any considerable distance, are termed holoptic, those in which the eyes in both sexes are separated by a broad frons, are termed dichoptic. Although many cases occur in which it is difficult to decide in which category to enroll them, the vast majority of Diptera can be allotted at first sight to either the holoptic or dichoptic class.
In holoptic Diptera through the eyes not being contiguous absolutely the whole distance from the vertex to the antennae, there is always a small inverted triangle just above where the eyes meet its apex pointing downwards and this is called the vertical triangle. At the point where the eyes separate again, a little above antennae is another moreof less triangular space with its apex pointing upwards, and this is called the frontal triangle. Obviously in flies with the eyes wide apart both triangles are merged in the frons and in such cases cannot be differentiated from it.
Below the antennae, as far as the mouth, is the face and posterior to the face below the eyes are the cheeks or gence.
The epistome or epistoma is a slightly vague term, but strictly speaking it means the mouth – opening and an indefinite space immediately contiguous thereto.
Many groups of Diptera possess a system of bristles about the head as well as on the thorax.
The thorax in the Diptera is seldom divisible into the component parts- prothorax, mesothorax and metathorax- but forms a uniform oval, oblong, elongateor sub quadrate central piece in the front of which the prothorax is sometimes discernible though in the Nemertocera especially in the Tipulidae, it is often obvious in the form of a circular flattened disc, termed by Osten Saken the collare.
The front part of the thorax when elongated to any appreciable extent is termed the neck, but in some families the head is too closely applied to the thorax for this to be visible. In the average Dipteron the head moves with great freedom upon the neck and is capable of being turned in all direction. The metathorax is hardly distinguishable from the mesothorax, especially on the upper side.
Across the center of the mesonotum in many Diptera is seen a more of less distinctly impressed line, often somewhat like a very widely opened V, finetest in the middle and ending on each side a little in front of the base of the wings. This is the transverse suture and it has a high morphological value, being very consistent when present.
The presutural depression is a small depression at each end of the transverse suture, usually triangular in shape.
The supra alar groove is a groove on the mesothorax just above the wing root and in many species bristles of taxonomic importance are found along its inner margin.
The scutellum is a projecting posterior lobe of the mesonotum, and a horny irregular rigde- like projection joining the scutellum on each side to the mesonotum is termed the scutellar ridge. It is some times but not usually, conspicuous.
Behind and below the scutellum the metanotum, a smooth and more of less swollen part, attaining its maximum development in the Tupulidae, in which it is usually much more conspicuous than the scutellum.
The halteres, which in Diptera replace the posterior wings, are small delicate organs consisting of a narrow moderately long stem, ending in an oval club, which occasionally is flattened or spatulate.
The sides of the thorax in many Diptera are distinctly subdivided by impressed lines known as pleural sutures, whilst in others such demarcations are not perceptible; when present they divide the sides into pleural spaces, which in some groups are well defined, whilst in others they are very indefinite. Most authors speak of them in general terms as “the pleurae”.
Few special terms are in use regarding he abdomen except that the external male genital organs are spoken of collectively as the hypophygium, whilst the female organs are known as the ovipositor. The eight segments of the abdomen are numbered from the base onwards, the usual terms used in the Tipulidad, where the coveing of this part of the bodyis represented by a distinct dorsal and ventral plate, being tergum and sternum respectively.
Description of the parts of the legs is followings:
The acetabulum is a minute joint attaching the coxa to the body; the coxa is the short first obvous joint which is united to the femur, the first long conspicuous part of the legs by another minute, ring like piece known as the trochanter. The tibia succeeds the femur and is the second conspicuous division, followed by the tursur or foot, which is invariably composed of five joints. These joints are numbered from the basal one, which is known either as the metatarsus or the tarsal joint.
Attached to the extremity of the ultimate tarsal joint are the ungues or claws, two curved hooks, and below these are two pads like cushions of pulvilli.
The anterior legs are the four front legs taken together, the posterior legs the four hind legs similarly considered.
The front margin of the wing is known as the costa and is carried right round the wing uninterruptedly, without any distinct or sudden termination near or just beyond the tip.
The auxiliary vein is the first vein below the costa and generally ends in the Tipulidae, between the middle (or just before the middle) and about three fourths the length of the wing. It is connected with the costa quite near its base by a short upright cross- vein, the latter known as the humeral cross – vein. The auxiliary vein is often known as the subcostal.
The next vein is a much longer one, running parallel with the auxiliary vein and ending some little distance beyond it, this is the 1st longitudinal vein.
In some few cases it fades away at the tip without turning either up or down. The auxiliary vein often lies so closely in front of the 1st longitudinal vein as to be imperceptible unless carefully looked for. In some few genera these two veins are actually united and in these cases the 1st longitudinal vein as the united are invariably called, generally approximates gradually to the costa, finally becoming merged in it and not turning distinctly up or down at the tip as in the typical forms.
The second longitudinal vein emerges from the first, usually some where near the middle of the latter, sometimes in a straight line at a mote of less acute angel, but more often in a gentle or distinct curve.
The portion or section of the 2nd longitudinal vein between its origin and the point of forking is called the proefuca; the two branches after forking, are the “anterior and posterior”, the “upper and lower”, or the “fore and hind” respectively.
The marginal cross vein is placed near the tip of the 1st longitudinal vein, joining it to the 2nd vein and dividing the marginal cell into two parts, often into about equal halves, these being known as the inner and outer marginal cell respectively. When the 2nd vein is forked this cross vein may unite with the upper branch or with the praefurca and it has a tendency to indistincness in many species.
The 3rd vein is simple in all the Oriental genera, except in the subfamily Ptychopterinae, in which its forked nature constitutes one of the principal characteristics of the group. It is nearly always present, any genus without it being most abnormal. In Toxorina it is altogether absent, the anterior cross vein connecting the 2nd and 4th longitudinal veins; in Mongoma (according to the interpretation of the venation) it is reduce to a short longitudinal connecting vein between the middle of the praefurca and the 4th longitudinal, thus not even approaching the margin of the wing.
The next vein is probably the most important of all in the wing: this is a short cross vein which almost invariably through out the Order connects the 3rd and 4th veins when both are present; and this vein, the anterior cross vein is in conjunction with the discal cell, technically a key to the venation in the whole order of Diptera. It divides the wing longitudinally into two nearly equal halves and no vein found in front of it in on group is ever found behind it in another.
The fourth longitudinal vein begins at the base of the wing and invariably encloses, or rather constructs the discal cell (when this is present) by being, in these case, always forked, the absence of the discal cell being only caused through the absence of a short cross vein between the two main branchesof the 4th vein and this cross vein usually called the discal cross vein, on account of its dominating the discal cell. Its upper or anterior branch forming the basal and anterior sides of this cell; the lower or posterior branch forming the posterior and outer sides of the cell.
The term central cross vein was sometimes used by Osten Sacken to designate the veins between the end of the praefurea (transversely across the wings length) and the posterior cross vein; in this case the short basal section of the 3rd vein, the basal part of the fork of the 4th and both sides of the discal cell works be include in addition to the two cross vein proper.
Not infrequently a genus of species is distinguished by the presence of an extra vein which constant in its occurrence. Such veins are called supernumerary. When they occur abnormally in individual specimens (often in one wing only) they are called adventitious and such occurrence are quite frequent, especially in the Eriopterini, in which their unexpected presence causes much difficulty to the beginner.
The Genital organs:
The internal genital organs consist in the male of two oval testes with short vasadeferentia, a well developed penis with accessory copulatory appendages, which are of the most diverse structure even in allide species of the same genus. In the female the ovipositor is remarkably uniform; there are a large number of egg- tubes, three spermathecae, paired accessory organs and no true bursa copulatrix.