health monitor without hostname

the monitor will use the IP

when the traffic came from your client the arrived with the URI requested

As FerCamargo stated, it will send the IP of the pool member it is sending the health monitor to. Please keep in mind that some servers tend to use strict RFC following and that IP is auto filled and the server could drop the request, not respond, or respond with a 400 series response. If you're going to use HTTP 1.1 you really should be adding a host field.

you say "what will be the hostname when traffic is sent to backend servers?". 

do you mean the traffic from the health monitor to the backend server?

it won't contain a hostname in that traffic, you didn't provide one in the Send String so the BIG-IP won't make on up.

this is quite easy to check if you have just one health monitor and no live traffic yet, run tcpdump on the BIG-IP and use something like this:

tcpdump -i 0.0 -nn host <backend server IP> -c 10 -X

you will see the Send String somewhere without any Host: header.

the Host header will contain ip addres of the pool member.