Introduction
The blood-feeding tendency of mosquitoes is part of their intrinsic character, as blood proteins are essential nutrients for egg production and reproductive fitness (Clements, 1992; Chaves et al., 2010). Many mosquito species are anthropophilic with respect to host preference and as a result, these species play an important role in the global transmission of the pathogens responsible for diseases such as malaria; filariasis and yellow fever (Jackman and Olson, 2002). Because transmission of pathogens occurs during blood-feeding, implementing personal protective strategies at times of the day or night when mosquitoes are most actively biting could be beneficial in terms of risk reduction. At the present time, very few studies have been conducted on the biting behavior of mosquitoes in Bangladesh (Ameen and Moizuddin, 1973; Ameen et al., 1982). These authors described the bionomics of several species of mosquitoes, but did not undertake a detailed investigation of temporal biting patterns of these species. To address this knowledge gap and to better define the temporal patterns of risk of exposure to mosquitoes that transmit pathogens, we conducted landing counts throughout the night once per month from March to July at a study site within the Jahangirnagar University.

1 Materials and Methods
1.1 Study area
Mosquito landing catches was done in a biodiversity rich protected area within the Jahangirnagar University (JU). JU campus is situated in between 30°16΄ N latitude and 90°52΄ E longitudes and is located 32 km north-west from the Dhaka city. The average height of this land is about 11 metres from the mean sea level. The campus consists of 282 hectares of diverse ecological habitats which include: open woodlands, grasslands as well as extensive wetlands.

1.2 Mosquito collection procedure
From March to July in 2010, landing catches were conducted at a single collection site. Mosquitoes were collected on one night per month from 18h00 to 06h00. Mosquitoes were collected in a manner similar to that described by Haddow (1954) using a mouth aspirator from the exposed legs and hands. During the study period T-shirt and short pants with no socks and shoes was wear. Mosquitoes were collected continuously, from exposed legs and hands over the first 50 minutes of each hour and rested for the remaining 10 minutes. Hourly catches of mosquitoes were kept separately in labeled cups, which were covered with fine nylon nets held with rubber bands.

1.3 Identification
After transport to the laboratory, the mosquitoes were anaesthetized using chloroform, all collected mosquitoes were identified to the species level using taxonomic keys (Christophers, 1933; Barraud, 1934; Bram, 1967; Reuben et al., 1994) and the total number of mosquitoes collected per hour and overall time points were recorded.

1.4 Data Analysis
The hourly counts of all mosquito species collected from 18h00 to 06h00 were pooled to examine the overall biting patterns throughout the night hours. Landing counts were only conducted on nights without rainfall or high winds as these conditions would disrupt mosquito host-seeking activity.

2 Results
2.1 Species composition
A total of 711 mosquitoes were collected during the study period. These mosquitoes belonged to three genera and seven species including: the filariasis vectors Culex (Cx.) quinquefasciatus Say (402, 56.54%), Mansonia (Mn.) uniformis Theobald (112, 15.75%), Mn. annulifera Theobald (88, 12.38%), and Armigeres (Ar.) subalbatus Coquillett (33, 4.64%). The Japanese encephalitis (JE) vectors Cx. tritaeniorhynchus Giles (51, 7.17%), Cx. vishnui Theobald (11, 1.55%) and the non-vector species Cx. hutchinsoni Barraud (14, 1.97%) were also captured during the study period.

2.2 Peak biting time of night
The largest number of host-seeking mosquitoes were collected between 24h00-03h00 (261, 36.71%) followed by 21h00-24h00 (230, 32.35%), and 18h00-21h00 (117, 16.46%). The fewest number of mosquitoes (103, 14.49%) were collected during 03h00-06h00 (Table 1).
 

Table1 Total number (%) of mosquitoes collected on human bait during various phases of night (18h00-06h00) Note: *Filariasis vectors, #Japanese encephalitis (JE) vectors

2.3 Landing density
The landing density or number of mosquitoes landing per hour (mph) ranged from 9.1 mph in July and 16.5 pmh in May (Figure 1). The average landing density throughout the study period was 11.85 mph. The highest and lowest landing density was found in May and July, respectively. The mean number of female mosquitoes landing per person per night (mpn) for filariasis and JE vectors was 12.7 and 12.4 respectively.
 

Figure 1 Monthly variation in biting density of mosquitoes in Jahangirnagar University campus

Biting behavior of Filariasis vector
Cx. quinquefasciatus was captured throughout the study period (Table 2) and the largest number of mosquitoes were captured from 24h00-03h00 (149, 37.06%) while the fewest (58, 14.43%) were collected during 03h00-06h00. Biting activity of Cx. quinquefasciatus increased gradually from 22h00 and peaked between 24h00 to 03h00 h and then decreased gradually. The maximum number of Mn. uniformis were caught from 24h00-03h00 (38; 33.93%) and the least during 03h00-06h00 (16; 14.29%). We did not observe a timing preference for host-seeking for Mn. uniformis. This vector of Brugian filariasis was found comparatively in much lower number than Cx. quinquefasciatus and this was more prevalent during the month of May. Mn. annulifera was collected throughout the night in variable frequencies. In general, there was also no time preference for biting shown by Ar. subalbatus. However, the temporal difference in biting behavior was apparent during different months (Table 2).
 

Table 2 Total number (%) of mosquitoes collected on human bait during different months Note: *Filariasis vectors, #Japanese encephalitis (JE) vectors

Biting behaviorof Japanese encephalitis vector
The two JE vector species, Cx. tritaeniorhynchus and Cx. vishnui were collected throughout the night at variable frequencies. Cx. tritaeniorhynchus were collected throughout the study period, but Cx. vishnui was found only in May, June and July (Table 2).Very few number of non-vector species Cx. hutchinsoni were captured in March and April.

3 Discussion
Knowledge of biting behavior of mosquitoes, especially of the vector species, is necessary to plan appropriate vector avoidance and control strategies. An advanced understanding of the biting behavior of mosquitoes is essential to clarify each species respective role in disease transmission in selected areas. During this study Cx. quinquefasciatus was found as the most predominant species. It was found actively pursuing hosts throughout the night with distinct peak activity in the middle of the night. The pattern of host-seeking behavior in our study markedly contrast that found in Bangkok (Sucharit et al., 1981). In Bangkok, the peak biting time of Cx. quinquefasciatus was between 22h00-23h00 with two other peaks after midnight at 01h00 and at 04h00 (Sucharit et al., 1981). This inconsistency in the results of various reports might be due to the differences in sleeping behavior of the host, microclimatic conditions, and the lunar cycle (Ghosh et al., 1985; Shriram et al., 2005).

The peak biting of Mn. uniformis was between 24h00-03h00 and a similar pattern was observed in a study conducted in Goa (Korgaonkar et al., 2012). In our study, Mn. annulifera were active throughout the night; with peak biting occur between 24h00-01h00, which was similar to the findings of Kumar et al., 1989.

The biting pattern of Ar. subalbatus was more or less consistent throughout the night. This contrasted the results of (Amerasinghe et al., 1994) who reported that Ar. Subalbatus was crepuscular in its feeding pattern. The discrepancy between the results of these studies may be due to the climatic conditions, especially temperature fall during the night and level of adaptation of the vector in the different countries (Gowda and Viayan, 1993).

Cx. tritaeniorhynchus was found to be the prime JE vector species in Jahangirnagar university campus. Temporal variability in the biting patterns of this species are well documented (Reuben, 1971, Reisen and Aslamkhan, 1976, Reisen and Aslamkhan, 1978). In our study, peak biting of Cx. tritaeniorhynchus occurred during 21h00-24h00. A similar result of nocturnal biting profile has been reported in India with the major peak towards midnight (Reuben, 1971), whilst a biphasic cycle with dusk and dawn peaks has been observed in studies from Pakistan (Reisen and Aslamkhan, 1978). A triphasic pattern with dusk, midnight, and dawn peaks has been reported in the Philipines (Reisen and Aslamkhan, 1976). All three types of biting patterns have been observed in Japan (Yajima et al., 1971). This variability has been attributed to different climatic conditions, especially temperature and humidity, in these different geographic regions (Reisen and Aslamkhan, 1978). The other JE vector species, Cx. Vishnui, was active throughout the night. While the non-vector species, Cx. hutchinsoni, was collected in negligible number in our study.

The landing density of mosquitoes varied from month to month likely influenced by climatic factors and human habits. Maximum biting density (16.5 mph) was found in May and lowest (9.1 mph) biting density was found in the July. In contrast, Gowda and Viayan (1993) observed a maximum biting density of Cx. quinquefaciatus in March (12.3 mph) and minimum in July (4.9 mph) in India.  These differences may be explained by the fact that we observed seven species where their study was consisting of only single species Cx. quinquefaciatus. It has been observed that biting density was higher in hot and rainy months (e.g., May) and climatic conditions (warmer temperatures and high humidity) may allow mosquitoes to live longer and parasite to develop faster (Noden et al., 1995).

Conclusion
In the present study, more than one third of all mosquitoes were host-seeking between 24h00-03h00. The main filarial vector Cx. quinquefaciatus peak biting time was between 24h00-03h00. It seems advisable that during this time period (and throughout the night), people should be advised to take precautions like the use appropriate repellents, and proper clothing as well as sleeping under a bed net to avoid vector-host contact.

Authors' contributions
MH carried out the fieldwork, participated in the analysis of data and drafted the manuscript. KB participated in the design of the study. KB, KMZR, MAR and AJH collaborated in writing the manuscript. All authors read and approved the final manuscript.

Acknowledgements
We thank Mr. Touhid Uddin Ahmed, Ex. Chief Scientific Officer, IEDCR, who kindly help to identify the mosquito species and Roknuzzaman Sarker for help during fieldwork. We acknowledge the Insect Rearing and Experimental Station (IRES) for proving technical and laboratory space during the study period. Dr. Robbin Lindsay from the Public Health Agency of Canada kindly provided editorial and scientific feedback on the manuscript.

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