Both pictures have the same radio set, namely the SCR-522, VHF command radio installed behind the pilot. The difference is one is mounted transverse and one fore and aft. The SCR-522 used a blade antenna (AN-104) often called an axe handle.
The wire antenna on fighters was often used for the other type of command set (SCR-274N) or a small navigation radio manufactured by Setchel Carlson and other manufacturers.
It is possible that this P-38 could have had both command sets (SCR-274N and SCR-522) installed but I think it more likely that the antenna wire was used for the navigation radio mentioned above.
The wire antenna is also seen on Mustangs coming through the canopy and going to the vertical stabilizer.
The P-38 would likely have had an IFF, identification friend or foe, radio that would identify it to interrogating radar sets as friendly as long as it transmitted the code of the day or mission. The IFF antennas were usually a wire rod type of antenna about 10 or so inches long. These thin wire antennas are hard to spot on most photos unless the shot was taken fairly close. The IFF set, or SCR-695, was usually in the right boom of the P-38 so the antenna was typically mounted close to the radio.
The AN-104 is easy to spot as it does look like an axe handle and tapers toward the end. Some aircraft, the P-61 for example, had two SCR-522's and thus two AN-104 antennas.
Here is a shot from the P-38 association's web site that shows the AN-104 VHF antenna mounted just in front of the nose gear:
Here is another shot from the P-38 associations web site showing the IFF antenna on the bottom of the right boom and the AN-104 on the bottom of the nose.
You can also see the wire antenna, or at least the insulator and tension spring, from the canopy to the vertical stabilizer on both aircraft.
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