**3. Functional morphology of Diptera**

Morphology and anatomy of the different life stages of fruit flies, particularly their characters useful for taxonomic purposes, have been described in detail by previous authors. But only those aspects relevant for an understanding of the group's developmental biology are considered here.

### **3.1 The adult**

Adults of Diptera have segmented bodies that include a head, thorax, and abdomen. The head has large eyes that cover the sides of the head with a small space between them in the front of the head. This helps them to see a wider area as they are flying (**Figure 5**). The adult body coloration of different dipteran species however varies from black through various shades of brown to orange or yellow. Yellow marks, particularly on the thorax, give many species a somewhat wasp-like appearance. This resemblance is particularly pronounced in certain *Bactrocera* subgenera and *Callantra* spp., which have petiolate abdomens, heavily fuscated costal stripes on the wings, and a jerky, wasp-like walk. The paired antennae each consist of three segments. Scanning electron microscope studies on *B. oleae* and *B. tryoni* indicate that the outer segment is covered with long cuticular spines interspersed with large numbers of chemosensilla of several distinct morphological types and functional significance [36–38].

The general structure of dacine flies is fairly typical of cyclorrhaphan Diptera. Male dacines, except those of some groups such as Gymnodacus, typically have a pair of combs (or pectins) comprised of stiff curved bristles on the lateral hind margins of the third abdominal tergite.

These combs function as stridulatory organs during courtship. Both sexes have a pair of tergal glands (ceromae) that open onto the surface of the fifth tergite. These

consist of dense groups of minute alveolae that secrete a waxy substance, which is spread onto the body and wings during preening [39]. In female dacines, abdominal segments 7–9 form the ovipositor, which is usually smooth and pointed but is serrated in some species. The apical segment has a number of chemosensilla; the most prominent are the preapical setae that arise from lateral grooves on either side of the segment [40]. These presumably play an important role in fruit discrimination.

#### **3.2 The egg**

The study of dipteran egg morphology has largely been confined to the description of surface features [41] mainly through transmission electron microscopy examination [42]. Typically, eggs of Diptera flies are elongate ellipsoidal in shape and thus have only a single primary axis (**Figure 6**). At one end, the egg bears a pedicel. The pedicel bears the micropyle and the aeropyles. Typically, the micropyle is located on the apex of the pedicel and may have a single- or multiple-openings. Among the majority of Diptera flies, the shape of the egg is usually elliptic or ovoid with elongated appearance. The end portion may be blunt, round or fusiform, subglobose with about 1 mm in size. The length-width ratio may vary depending on the species. In several species of Sarcophagidae, however, the egg length may be 2.5 cm or longer [43]. Egg coloration widely varies within this order, and may range from pale (shortly after oviposition) to dark (towards embryo development). The micropyle may be arranged in a similar manner as in most nonfrugivorous species. The pedicel may be only a slight outgrowth or an elongated stalk nearly as long or longer than the overall length of the egg [44].

Dipteran eggs usually develop inside the ovariole during which the pedicel begins to orient towards the terminal portion of the ovary. It is understood that the basic functions of fertilization and oviposition are facilitated by this orientation process. As observed by [45], fertilization usually occurs as the egg moves

**Figure 5.** *Dorsal view of adult of Dacine fly.*

**153**

**Figure 7.**

*Phylogeny and Functional Morphology of Diptera (Flies) DOI: http://dx.doi.org/10.5772/intechopen.90421*

inserted into tissues of the host plant [48].

the egg [46].

**3.3 The larvae**

towards the middle portion of oviduct through the micropyle. The gonopore exists within the basal part, near the end of the aculeus (the part inserted into the tissues of the plant during oviposition). Shortly after oviposition, the process of embryogenesis commences. During the process of embryogenesis, the developing head of the embryo then begins its orientation towards the pedicel. In many species, however, just before eclosion, the embryo is said to rotate 180° and then leaves the egg through the basal end. This process makes it possible for the embryo to be positioned so that the plant tissue can be easily encountered upon eclosion from

The surface of a matured egg of Diptera may appear smooth or rough (due to the presence of microsculptures containing chorion derivatives) Also, the egg surface may have polygonal reticulations (mass-relief-type ridges) which represent the follicle cells outlines and are responsible for the lay down of the chorion [47]. For *Tephritis baccharis* (Coquilett), the reticulations may appear more prominent and may bear further structural decorations. For *Aciurina thoracica* (Curran), the egg surface may develop as rough at the end of the pedicel, and then diminish to a smooth surface close to the basal end. Previous studies hypothesized that the pedicel usually needs greater structural support for protecting the aeropyles and associated channels of respiration from deformation. This is because the pedicel portion of the egg is exposed to facilitate exchange of gases, while the basal end is

Larvae are the small wormlike early stages of Diptera flies, usually called maggots. Larvae of lower Diptera range in length from only a few millimeters to many centimeters, depending on the species, and are usually distinguished by having a conspicuous head capsule with opposable mandibles that move in a pincer-like horizontal plane. After eggs hatch, larvae begin to feed on the decaying materials within which they were laid (**Figure 7**). Larvae consume as much food as possible in order to store energy and nutrients for the upcoming pupal stage. Three free-living instars exist for tephritid fruit flies. The only known exceptions are *Urophora jaceana* (Herring) and *Urophora cardui* (Linnaeus), in which the first instar remains in the egg and exits as a second instar. The external anatomy of the larvae of frugivorous tephritids has been examined in detail, and at least partial descriptions based primarily on scanning electron micrographs for 25 species have been available.

*Blowfly larva,* Chrysomya bezziana *(Calliphoridae). (A) Complete larva; (B) anterior spiracle; (C)* 

*cephalopharyngeal skeleton; (D) spines; (E) caudal end with pair of spiracular plates.*

**Figure 6.** *Matured eggs of Tephritid fly exposed from an infested fruit.*

*Phylogeny and Functional Morphology of Diptera (Flies) DOI: http://dx.doi.org/10.5772/intechopen.90421*

towards the middle portion of oviduct through the micropyle. The gonopore exists within the basal part, near the end of the aculeus (the part inserted into the tissues of the plant during oviposition). Shortly after oviposition, the process of embryogenesis commences. During the process of embryogenesis, the developing head of the embryo then begins its orientation towards the pedicel. In many species, however, just before eclosion, the embryo is said to rotate 180° and then leaves the egg through the basal end. This process makes it possible for the embryo to be positioned so that the plant tissue can be easily encountered upon eclosion from the egg [46].

The surface of a matured egg of Diptera may appear smooth or rough (due to the presence of microsculptures containing chorion derivatives) Also, the egg surface may have polygonal reticulations (mass-relief-type ridges) which represent the follicle cells outlines and are responsible for the lay down of the chorion [47]. For *Tephritis baccharis* (Coquilett), the reticulations may appear more prominent and may bear further structural decorations. For *Aciurina thoracica* (Curran), the egg surface may develop as rough at the end of the pedicel, and then diminish to a smooth surface close to the basal end. Previous studies hypothesized that the pedicel usually needs greater structural support for protecting the aeropyles and associated channels of respiration from deformation. This is because the pedicel portion of the egg is exposed to facilitate exchange of gases, while the basal end is inserted into tissues of the host plant [48].

#### **3.3 The larvae**

*Life Cycle and Development of Diptera*

longer than the overall length of the egg [44].

**3.2 The egg**

consist of dense groups of minute alveolae that secrete a waxy substance, which is spread onto the body and wings during preening [39]. In female dacines, abdominal segments 7–9 form the ovipositor, which is usually smooth and pointed but is serrated in some species. The apical segment has a number of chemosensilla; the most prominent are the preapical setae that arise from lateral grooves on either side of the segment [40]. These presumably play an important role in fruit discrimination.

The study of dipteran egg morphology has largely been confined to the descrip-

tion of surface features [41] mainly through transmission electron microscopy examination [42]. Typically, eggs of Diptera flies are elongate ellipsoidal in shape and thus have only a single primary axis (**Figure 6**). At one end, the egg bears a pedicel. The pedicel bears the micropyle and the aeropyles. Typically, the micropyle is located on the apex of the pedicel and may have a single- or multiple-openings. Among the majority of Diptera flies, the shape of the egg is usually elliptic or ovoid with elongated appearance. The end portion may be blunt, round or fusiform, subglobose with about 1 mm in size. The length-width ratio may vary depending on the species. In several species of Sarcophagidae, however, the egg length may be 2.5 cm or longer [43]. Egg coloration widely varies within this order, and may range from pale (shortly after oviposition) to dark (towards embryo development). The micropyle may be arranged in a similar manner as in most nonfrugivorous species. The pedicel may be only a slight outgrowth or an elongated stalk nearly as long or

Dipteran eggs usually develop inside the ovariole during which the pedicel begins to orient towards the terminal portion of the ovary. It is understood that the basic functions of fertilization and oviposition are facilitated by this orientation process. As observed by [45], fertilization usually occurs as the egg moves

**152**

**Figure 6.**

**Figure 5.**

*Dorsal view of adult of Dacine fly.*

*Matured eggs of Tephritid fly exposed from an infested fruit.*

Larvae are the small wormlike early stages of Diptera flies, usually called maggots. Larvae of lower Diptera range in length from only a few millimeters to many centimeters, depending on the species, and are usually distinguished by having a conspicuous head capsule with opposable mandibles that move in a pincer-like horizontal plane. After eggs hatch, larvae begin to feed on the decaying materials within which they were laid (**Figure 7**). Larvae consume as much food as possible in order to store energy and nutrients for the upcoming pupal stage. Three free-living instars exist for tephritid fruit flies. The only known exceptions are *Urophora jaceana* (Herring) and *Urophora cardui* (Linnaeus), in which the first instar remains in the egg and exits as a second instar. The external anatomy of the larvae of frugivorous tephritids has been examined in detail, and at least partial descriptions based primarily on scanning electron micrographs for 25 species have been available.

#### **Figure 7.**

*Blowfly larva,* Chrysomya bezziana *(Calliphoridae). (A) Complete larva; (B) anterior spiracle; (C) cephalopharyngeal skeleton; (D) spines; (E) caudal end with pair of spiracular plates.*

By comparison, an atlas of immature morphology based on the third instar of 34 economically important species has been developed. The segments of the maggot typically bear spines in regular patterns (**Figure 7**) and the larvae of some species may possess structures that vary from simple setae to large protuberances. Several other structures including the median oral lobe, the lateral spiracles accompanied by a variable number of sensilla associated with the sensory organs of the gnathocephalon have been newly identified for various frugivorous species.

Tephritids have distinct anterior and posterior spiracles. Modern scanning electro-microscopy have aided in the location of the lateral spiracles along the thoracic and abdominal segments, as well as along the caudal segment that bears the posterior spiracles [49]. The lateral spiracles, which are always present along the lateral and anterior portion of a segment, have been known to have a varying number of campaniform sensilla associated with it around the posterior end of the spiracle. The number of sensilla may range from one (as in some *Aciurina* and *Trupanea* species) to as many as four (as in *Stenopa affinis* Quisenberry). When more than one spiracular sensilla is present, they are typically arranged adjacent to the spiracle, along a dorso-ventral axis.

#### **3.4 The pupa**

Dipteran pupa becomes more impervious to the surrounding environmental conditions and the larva becomes morphologically reduced and evolved to feed on nutrient-rich substrates; flies as a whole may occupy a broad range of trophic niches. The puparium is the hardened, penultimate larval integument of the developing fly (**Figure 8**). It is remarkable in its external morphology in tephritid fruit flies. When the third instar larva is ready to pupate, it leaves the medium, and its anterior spiracles evert, body shortens and ceases to move and it attaches to a firm substrate. The cuticle then transforms into a puparium, which is initially soft and white, but soon hardens, turning tan and eventually becomes brown and with bristle. Shortly after the puparium forms, then metamorphosis then takes place. The prepupal integument is shed and adheres to the inner wall of the puparium. The pupa forms within the puparium after the prepupal molt. The pupa develops independently of the puparium and has bilobed thoracic spiracles for respiration. The larval tracheae adjacent to the anterior and posterior spiracular openings remain open, thus allowing for gas exchange for the developing pupa within the puparium.

According to Ref. [50], there exists a pre-puparial stage in which the mouthparts contain series of invaginations. During this stage, the integument typically assumes a waxy appearance, but the processes of tagmentation (hardening and darkening of the integument) are delayed. The darkening of the integument may be triggered

**155**

*Phylogeny and Functional Morphology of Diptera (Flies) DOI: http://dx.doi.org/10.5772/intechopen.90421*

health, agricultural productivity and food security.

University for Development Studies, Tamale, Ghana

\*Address all correspondence to: benbadii@uds.edu.gh

provided the original work is properly cited.

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Author has no conflict of interest.

altitudes [51].

rhythm.

**4. Conclusion**

**Conflict of interest**

**Author details**

Benjamin Kongyeli Badii

by changes in the prevailing environment, by overwintering prepuparia as in the case of certain *Neaspilota* and *Urophora* species, particularly those found at higher

Eclosion marks the end of pupation and the beginning of the adult life. The insect cracks open the puparium anteriorly and laterally at its seams and emerges from the pupal case. This almost invariably occurs around dawn, when leaves are still dump with dew, and the emerging fly can fold its new wings and harden its cuticle without the risk of desiccation. The timing of this is controlled by circadian

These phylogenetic and morphological reviews of Diptera provide an evolutionary framework for future comparative work on species that are critically important to both society and science. The order Diptera has been divided into two or three suborders: the monophyletic Nematocera and the paraphyletic Brachycera, with the latter being divided further into the Orthorrhapha and Cyclorrhapha. The living dipteran species have been classified into about 10,000 genera, 150 families, 22–32 superfamilies and 8–10 infraorders. The typical dipteran body morphology is reflected in its life cycle which includes a series of distinct stages or instars; consisting of a brief egg stage, three or four instars, a pupal stage of varying length, and an adult stage that lasts from less than 2 hours to several weeks or even months. The species-richness, morphological variability and ecological diversity of this order of insects dictate the economic importance of the group to man and reflects the range of organisms in the order. Future work will focus on contributions and progress in understanding of the bioecological processes and economic impacts of dipteran flies in human life especially in relation to

**Figure 8.** *Pupal forms of Diptera.*

*Phylogeny and Functional Morphology of Diptera (Flies) DOI: http://dx.doi.org/10.5772/intechopen.90421*

by changes in the prevailing environment, by overwintering prepuparia as in the case of certain *Neaspilota* and *Urophora* species, particularly those found at higher altitudes [51].

Eclosion marks the end of pupation and the beginning of the adult life. The insect cracks open the puparium anteriorly and laterally at its seams and emerges from the pupal case. This almost invariably occurs around dawn, when leaves are still dump with dew, and the emerging fly can fold its new wings and harden its cuticle without the risk of desiccation. The timing of this is controlled by circadian rhythm.
