Syrphids (commonly known as hoverflies or flower flies) are one of the most diverse groups of dipterans, with more than 6300 described species, distributed in about 200 genera. The family Syrphidae is taxonomically divided into four subfamilies: Microdontinae, Eristalinae, Pipizinae and Syrphinae, being the Eristalinae the most diverse and speciose group, but also the only one not sharing a close common evolutionary ancestor. As with many other insect groups, most knowledge of the biology and ecology of Syrphidae comes from the study of the adult stage, with little or no knowledge of the preimaginal stages. This lack of information hinders the resolution of key phylogenetic and evolutionary issues, as well as the implementation of conservation and protection measures for endangered species and others that play very important ecological roles. Therefore, the main objective of this thesis is to increase this knowledge, both from a basic and applied research point of view. For this purpose, two of the most important phylogenetic clades of eristalines were selected: the basal tribe Merodontini (Sections I and II) and the highly specialised tribe Eristalini (Section III). The aim of Section I is to extend and revise the knowledge of the larval morphology and taxonomy of the genera Merodon Meigen, 1803 and Eumerus Meigen, 1822. The output is the preimaginal description of a total of 17 taxa, including morphological studies of third instar larvae, puparia, and head skeletons. The section is divided into two chapters focusing on each of the genera. Chapter I includes the study of the evolutionary lineages M. desuturinus and M. albifrons of the genus Merodon. First, four species belonging to the capi speciescomplex of the planifacies subgroup of M. desuturinus lineage were described: M. capi Vujić & Radenković, 2020, M. roni Radenković & Vujić, 2020, M. levis sp. nov. and M. reenensis sp. nov. The larvae were found feeding inside the epigeal bulbs of the South African host plant Merwilla plumbea (Lindl.) Speta. Due to the very similar morphology and the fact that the species-complex includes some cryptic taxa, morphometric analysis of their larval morphology in syrphids was used to obtain their morphological characterisation. This is the first time that this type of analysis has been applied to syrphid larval morphology. In addition, first chapter also contains descriptions of eight species belonging to the constans-group of M. albifrons lineage: M. adjaricus sp. nov., M. analis Meigen, 1822, M. caucasicus Portschinsky, 1877, M. chrysotricos Vujić, Radenković & Likov, 2020, M. dhzalitae Paramonov, 1926, M. gudaurensis Portschinsky, 1877, M. resani sp. nov. and M. triangulum Vujić, Radenković & Hurkmans, 2020. These species were collected feeding on bulbs of the host plants Leucojum vernum L., and different species of the genus Galanthus L. in four countries of the Eastern Mediterranean. The results significantly enlarge the number of available descriptions of the M. albifrons lineage, which becomes the group with the highest number of species with preimaginal stages described in the genus. The second chapter includes five descriptions of the genus Eumerus, two of which were collected in the eastern part of the Iberian Peninsula and the other three in South Africa. The first two species were found feeding inside the stems of Cistanche phelypaea (L.) Cout.: E. larvatus Aracil, Grković & Pérez-Bañón, 2023 and E. amoenus Loew, 1848. The morphology of the two species is completely different, especially in the head skeleton and posterior part of the body. These remarkable differences, suggest different feeding habits for each of them. Two different morphological groups are then recognised: one that includes the traditional and widely recognised genus morphology corresponding to saprophagous species, and another that includes truly phytophagous species. The other three species described in Chapter II are saprophagous species collected feeding inside the decaying stems of Aloe candelabrum A. Berger, i.e. E. compactus van Doesburg, 1966 and E. rufotibialis sp. nov., and inside epigeal bulbs of M. plumbea, i.e. E. astropilops Hull, 1964. The results increase the knowledge of the genus Eumerus in the Afrotropical region, where it is particularly diverse and the amount of information is very scarce, especially regarding larval biology and morphology. The preimaginal morphology of these species fits the saprophagous type previously described. Diagnostic characters for each species have been found and indicated. The main aim of Section II is to provide new information on the life cycle, the geographical distribution, and the population dynamics of several species of the genera Merodon and Eumerus. To this end, the section has been divided into two chapters, one focusing on two species of the genus Merodon, described in the previous section, and the other on two species of the genus Eumerus, one with a phytophagous and one with a saprophagous larval type. Chapter III aims to investigate the life cycle and the relationship between the adult stage and the host plant of two Afrotropical species studied in Chapter I: M. capi and M. roni. Therefore, a morphological description of the mouthparts of the adults and a pollen analysis were performed; a third South African species, which also belongs to the M. desuturinus lineage, was used as a comparison for the analysis, i.e. M. drakonis Vujić & Radenković, 2018. The results showed that the species do not have very developed mouthparts, and very low amounts of pollen were found in the body and gut. This information suggests that both species do not visit the flowers of M. plumbea, and the relationship with the host plant is mainly limited to the preimaginal stage. In conclusion, the species seems to be univoltine, with the larvae developing inside the epigeal bulbs, and moving through the external part to pupate in the soil. Chapter IV deals with the study of a phytophagous Eumerus species (described in Section I) studied in situ in the field, and a saprophagous species studied under laboratory conditions using the “age-stage, two-sex life table theory”. The phytophagous species, E. larvatus, is an endemic species from the eastern Iberian Peninsula. It is a univoltine species, with a very short and atypical winter flight period. The larvae feed inside the stems of C. phelypaea during spring and midsummer, and remain buried in the soil throughout autumn, until early winter when they move on to the surface to pupate. On the other hand, in the fourth chapter, Eumerus obliquus (Fabricius, 1805), was reared for the first time under controlled conditions using an artificial larval food based on soaked grains. The age-stage two sex method has proved to be a useful technique for studying the demographic and population parameters of this species, which is in expansion. The results show a developmental time of 2, 14.80, 15.90 and 22.3 days for egg, larva, pupa, and adult, respectively. The highest mortality was found in the first instars of the larval stage, accounting almost 50 %. The average fecundity is approximately 260 eggs per female, oviposition begins when females are four days old, and the average hatchability is 55 %. The aim of Section III is to study of the effect of pupal cold storage (5 ºC) on five eristaline hoverflies with high potential for use as pollinators in commercial crops. The first chapter of the section (Chapter V) presents the results obtained with two species for which there are no standardised protocols for mass rearing: Eristalinus taeniops (Wiedemann, 1818) and Myathropa florea (Linnaeus, 1758). In the second (Chapter VI), three species with well-established massive rearing protocols were analysed: Eristalinus aeneus (Scopoli, 1763), Eristalis arbustorum (Linnaeus, 1758), and Eristalis tenax (Linnaeus, 1758). To achieve this aim, eight cold treatments were applied in both chapters: constant cold (CC) storage for 5, 10 and 15 days vs. fluctuating regime treatment (FTR) for 15, 20 and 30 days, together with a control treatment without cold exposure (25 ºC). Furthermore, in chapter VI an analysis of the flight capability of the adults emerged after cold storage was carried out using flight-mill devices. A summary of the main results of this section is presented below. The parameters studied varied in a similar way for the five species analysed, then in all cases, the development time increases exactly with the number of days that the pupae are exposed to cold conditions. The emergence and viability rates, decrease with increasing exposure time for both constant and fluctuating regime treatments, but the morphological alteration rate increases with increasing cold exposure time. The fluctuating regime treatment considerably increases emergence and viability rates and decreases morphological alteration rates. On the contrary, the sensitivity of each species to cold exposure is different, with E. tenax and E. aeneus being the most resistant species, while E. arbustorum and E. taeniops are very sensitive to cold exposure and present worse rates than the others. In the case of M. florea, it presents more sensitive rates (like the second group) for the constant cold treatments, but it seems to be very resistant when the fluctuating regimes are applied, being the one with the highest rates in the FTR4-30D treatment. Considering a minimum viability rate of 80 % as a valid (usable) treatment, the best treatment for E. arbustorum, would be CC-5D (88 % of viability) and although there are several valid options for E. tenax, the most useful would probably be the FTR4 treatments for the period requested (not more than 20 days). Also, for E. aeneus most treatments have high viability rates but, in this case the best would be FTR4 for a period of 15D. For M. florea, FTR4-20D was the best treatment, but for E. taeniops all treatments were above the minimum threshold, the closest being FTR2-15D with a viability rate of 77 %. Finally, the analysis of flight capability was carried out for E. aeneus, E. arbustorum, and E. tenax using three different treatments for each species. The results showed that short periods of cold exposure did not affect the flight capacity of the adults, but longer periods of exposure (15 or 20 days) reduced the flight capacity; in the case of E. aeneus, the longer exposure treatment resulted in total inability to fly. However, the number of replicates performed in the study was not enough to carry out a robust statistical analysis, so further research is needed to determine the effect of pupal cold storage on adult flight capability. / Para la realización de esta tesis se ha contado con la financiación del Programa Propio del Vicerrectorado de Investigación y Transferencia de Conocimiento para el fomento de la I+D+i en la Universidad de Alicante 2018, bajo un contrato de investigación predoctoral (UAFPU2019-03).