The lowest number of published articles occurred in 2014 (n = 244; 8.83%); however, after 2014 there was an increase in the number of publications, with 2019 being the year with the highest number (n = 302; 10.93%). The journal with the highest number of published articles on Odonata was Zootaxa, with a total of 284 publications (10.27%), followed by the International Journal of Odonatology (226 publications; 8.18%), and Odonatologica (202 publications; 7.31%). Other important journals (an additional 497 journals) included fewer than 56 articles each. The five most prominent words highlighted in the keywords cloud were ‘Odonata’ (weight = 1.00), ‘dragonfly’ (weight = 0.95), ‘damselfly’ (weight = 0.56), ‘species’ (weight = 0.53) and ‘Zygoptera’ (weight = 0.38).

There was an increase in the number of publications and the frequencies and trends changed over the time analyzed. Although there are years with low production during the period analyzed, the trend towards an increase in the number of studies on dragonflies is maintained when compared with previous analyses [36] but reaches a plateau after 2017.

Zootaxa, International Journal of Odonatology and Odonatologica, are the journals with the highest number of papers published on Odonata. All are international, specialized, and peer-reviewed journals, but there are differences between them, which can be used to better interpret the results. Zootaxa is a broad scope journal, with a preference for papers in zoology (any animal taxa), mainly focused on the systematic review of groups and/or description of new taxa, in a fast, high-quality format. Its wide scope, frequency of publication, and review system have allowed this journal to have a positive impact on the advancement of knowledge of many groups, including Odonata [42]. On the other hand, it was expected that the International Journal of Odonatology and Odonatologica, specialized journals in Odonatology, would also have many publications. Both accept any type of research related to dragonflies, so their spectrum includes broader fields of research such as ecology, conservation, ethology, and reproduction.

The ten countries with the most publications were the USA (n = 346), Brazil (n = 272), China (n = 204), Germany (n = 135), Japan (n = 132), Canada (n = 122), Sweden (n = 121), France (n = 110), Mexico (n = 103) and India (n = 83). Several other countries had >10 publications, indicating the global interest on the Odonata as a target organism for research.

Regarding the countries with the largest number of publications, for the American continents, the USA (first place) and Brazil (second place) remain the countries with the largest number of publications. This reaffirms the importance of the work developed by these two countries, which have a broad tradition of research contributing considerably to the knowledge of Neotropical dragonflies [36]. The case is similar in the African context, where the countries that maintain the highest number of publications are South Africa and Algeria, places where leading odonatologists have been established for many years. Likewise in Europe and Asia, the greater number of publications are in countries with a long history and tradition of Odonata research such as Germany in Europe, or China and Japan in Asia [36].

Most studies focused on Odonata ecology (n = 717), followed by studies on taxonomy (n = 584), behavior (n = 576), morphology (n = 343) and ecological monitoring (n = 207).

The studies show a great diversity of research areas in which dragonflies have been used as research targets. However, it was expected that the largest number of publications would be focused on ecology and biodiversity studies, because many journals publish articles on Odonata that are not specific to taxonomy or phylogeny. Miguel et al. [36] found the same trend worldwide. Moreover, areas such as ecology and behavior are constantly growing, so periodically new metrics, approaches and methodologies are published, generating interest to replicate them in different parts of the world [46,47]. Ecology is the main research focus on the American and African continents. In Asia and Oceania, taxonomy is the main type of study, while in Europe, behavior studies stand out.

Although studies in genetics have increased, there are disadvantages in their replicability, especially in developing countries, because of both financial and equipment limitations [36]. Finally, topics of decreasing scientific interest and funding are associated with fewer publications (e.g., natural history and basic species biology).

The focus of research on each continent is varied and likely related to who lives where. For example, the fact that the Americas contain the largest number of researchers in ecology, behavior and taxonomy is directly related to the establishment and growth of different research groups, mainly in Argentina, Brazil, Colombia, the United States, and Mexico (e.g., Dragonfly Society of the Americas and Sociedad de Odonatologia Latinoamericana) [36,48,49,50,51]. Asia and Oceania each show the same research patterns, mainly taxonomy, morphology, and ecology. In all three continents, there is a history of taxonomic studies which is maintained today. In Europe, the main research focus is behavior, ecology, and morphology. European odonate biodiversity has been well known for a century, and only after more than 100 years was a new species described there [52]. Africa is the only continent where the main lines of research are also focused on conservation. This is certainly related to the studies developed by several research groups that are focused on conservation, especially in South Africa, such as the research group led by Michael J. Samways and John P. Simaika. M.J. Samways has published extensively on various aspects of Odonata ecology and conservation [53], especially regarding landscape ecology and insect conservation in general [54]. J.P. Simaika has more than a decade of experience working in rivers, lakes, wetlands and artificial ponds in Africa. Their research has led of public conservation policies at the international level [54].

The highest number of published articles on Odonata were conducted in the field, i.e., in streams (n = 668), ponds (n = 437), rivers (n = 318), and lakes (n = 278), but many were also part of laboratory experiments (n = 364) or involved fossilized material (n = 125). Markedly fewer studies have been conducted in pools (n = 42), reservoirs (n = 20), mesocosms (n = 10), and plants (n = 3).

Because of the strong relationship between dragonflies and aquatic environments, it is reasonable that research on Odonata focused on some of these environments. Most of the research has been conducted in lotic environments, such as rivers and streams, systems that are under constant anthropogenic threats [55]. The type of impact, as well as the degree of intensity, affects the complex dynamics of functioning and interconnection in this habitat, generating serious effects on their health as well as on the biodiversity that inhabits them [56]. Because of this, numerous environmental laws across the world stress the evaluation and monitoring of lotic bodies as a priority [1,57].

Of the total articles on Odonata, 982 involved the Zygoptera, 946 Anisoptera and 15 the Anisozygoptera. Another 756 studies were focused on both main suborders, i.e., Anisoptera and Zygoptera. Most studies (n = 1662) were focused only on adults, 714 only on the nymphs, 160 on adults and nymphs, and 40 on nymphs and eggs. It is noteworthy that several articles focused on more than one life stage. Most articles (n = 2381) used species-level taxonomy.

Most studies focused on Odonata, including the suborders Anisoptera and Zygoptera in their analyses. Numerous studies included both suborders to compare the responses among their species [23,44]. When a single suborder was analyzed, the Zygoptera and Anisoptera were used with similar frequencies. Thus, there was no clear pattern or preference for a specific suborder. One reason for this tendency is the well resolved taxonomy in both suborders [43], because a resolved taxonomy is the basis for asking different questions in areas such as evolution, systematics, or ecology.

Adults or larvae were the stages most commonly used in research. Presumably, this is because adult Odonata are visible in the field, their collection requires only an insect net, excellent taxonomic keys for most species are available and easily accessible, and the adults can be identified to the species level [3,4,9]. This is partially true for larvae, the second stage with the most publications, particularly in regions with a long tradition in larval dragonfly research [58]. On the other hand, in regions where this tradition is more recent, there are fewer larval studies [36]. Furthermore, only the larvae of 1/3 of the Odonata species are described [59] and larvae detection and transportation from the field to the laboratory is more complex [36]. Research studies on both adults and larvae are numerous, which is especially useful when one may want to compare responses between the two phases to provide a more comprehensive analysis [27]. There has also been research on three or more phases, presumably when one wants to evaluate ontogenetic development, for which the collection of eggs and the development of larvae are needed [60]. This also applies in the case of larvae and exuviae and larvae and adults, where the association of the different stages is important for taxonomic description [61].

Despite not being an official life stage, research on exuviae shows great potential for tracing the route of environmental contaminants [62] or for adults that are difficult to find or capture [63]. Exuviae also provide proof of life cycle completion at particular habitats [62]. However, exuvia research requires investing much field time, considerable care in transportation and storage of such fragile specimens, and identification problems similar to those with larvae. Incidentally, the results reinforce the evidence that shows how different life stages in Odonata are useful for evaluating different research questions. Finally, it is not surprising that fossil studies are numerous in Odonata because there are many odonate fossil records, which provide the basis for studies of evolutionary relationships within the order [64].

Species is the taxonomic level most often used in most studies, but genus, family, and order are used as well. Identification to species is important because it facilitates more accurate information about a particular taxon. However, this requires more time, taxonomic expertise, and financial resources, which is a disadvantage for projects that need an immediate response or are underfunded [65]. Therefore, there is much discussion in the literature regarding the preferable taxonomic level for studies. For example, Jansen et al. [66] point out that it is challenging to classify organisms into higher-level taxonomic groups, such as families, because many family-specific features may not be distinguishable.

Although most Odonata papers in the WoS database use the species level of taxonomic resolution, Mendoza-Penagos et al. [35] demonstrated that the family level provides an effective tool odonate for biomonitoring of tropical streams by providing ecologically meaningful information. Future studies should evaluate the costs and benefits of diagnosing impacts by comparing multiple taxonomic levels against a common disturbance gradient [67,68].