Tiberius Julius Caesar Nero (Gemellus) (b. A.D. 19, d. A.D. 37/38) was one of the twins born to Drusus Caesar, the son of Tiberius, in the ill-fated year of Germanicus’s death. Gemellus’s brother died in infancy, but lent the nickname Gemellus (“The Twin”) to our subject.  Gemellus’s father, Drusus, died in mysterious circumstances when Gemellus was only four years old, possibly the first victim of the scheming Praetorian Prefect, L. Aelius Seianus. The boy’s life thereafter appears to have been largely uneventful, since he received minimal official recognition from his grandfather, Tiberius. He lived out his childhood in the shadow of his cousins Drusus and Nero Caesar, and a major landmark of his youth, the assumption of the toga virilis, was not celebrated until he was eighteen years of age, four years later than was habitual. At some time he joined the Arval Brotherhood, but this was hardly a meteoric elevation.  Only with the fall of the house of Germanicus in the mid-30s did an opening appear for Gemellus, who was by then approaching maturity. But, unfortunately for him, the ascendancy in Tiberius’s estimation of Germanicus’s surviving son, Gaius (Caligula), meant more time in the shadows. Tiberius’s attitude toward his natural grandson in the years A.D. 31-37 was the subject of intense speculation among the ancients. 
On Tiberius’s death on 16 March, A.D. 37, Gaius came to power with the support of the Praetorian Prefect, Macro. When Tiberius’s will was read in the senate, it contained a surprise: the nomination of Gemellus as chief co-heir with Gaius.  This act catapulted Gemellus to the fore and probably cost him his life. Gaius moved swiftly to negate the threat. He had Tiberius’s will nullified (on what legal basis is unknown) and was, initially, placatory in his dealings with his cousin. He adopted Gemellus as his son and had him declared princeps iuventutis, both procedures long since reserved for favored princes in line to succeed reigning emperors. However, by the end of the year, or early in A.D. 38, Gemellus was murdered on Gaius’s orders.  His short life amply illustrates how the ill-defined legal position of the Augustan princeps and the uncertain modes of nominating successors employed by the emperors proved fatal for so many members of the Julio-Claudian family.
There is no single work on the life of Gemellus. Rather his life and importance are assessed variously in the main works listed in bibliographies of the DIR entries for Tiberius and Gaius (Caligula). Do note, however:
Gardthausen, V. “Iulius” (no. 156). Real-Enzyclopädie 10.536-37 (1919).
Herz, P. “Die Arvalakten des Jahres 38 n.Chr. Eine Quelle zur Geschichte Kaiser Caligulas.” Bonner Jahrbücher 181 (1981): 89-110.
 The sources for Gemellus’s life are meagre. He is referred to allusively in the major sources for the period — Tacitus, Suetonius, Dio, Josephus, and Philo — but he was very much a minor character and so largely passed over in silence. Most of the relevant passages are referred to in the following notes. See also the evidence collected in PIR2 I 226.
 Toga virilis: Suet. Gaius 15.2; Dio 59.8.1. Arval Brotherhood: He is reported in the Acta Fratrum Arvalium as having been replaced on 24 May 38 by P. Memmius Regulus, cf. Smallwood, Documents Illustrating, no. 3, lines 32-36. Incidentally, the cooptation date of Gemellus’s replacement may suggest that Gemellus died sometime between February and May A.D. 38, rather than later in A.D. 37 as is generally believed, since it appears that cooptations into the Arval Brotherhood were staged in February, May and June of each year; see further, Herz, “Die Arvalakten”; J. Scheid, Romulus et ses confrères. Le collège des frères arvales, modèle du culte public dans la Rome des empereurs (Rome, 1990), 187-96. Note that Josephus (AJ 18.166, 187-88, 191) depicts Herod Agrippa ignoring Gemellus in favor of Gaius, despite Tiberius’s injunction that he court Gemellus. This tradition reinforces Gemellus’s weak position in these years.
 Tacitus (Ann. 6.46) portrays Tiberius as inclined to Gemellus due ties of blood but aware of his youth and fearful for his welfare after his own death. Josephus (AJ 18.205-14, 218-222) recounts the folksy story that Tiberius resolved to nominate as successor whichever of his grandsons, Gemellus or Gaius, came to him first on an appointed day. Gaius, of course, showed up first and Tiberius, aware of Gemellus’s weak position, petitioned Gaius to spare his grandson, to no avail. Philo (Leg. 23-25) presents Tiberius as favoring Gemellus and actually planning to have him succeed, had the aged emperor not died before this plan could be effected. Suetonius (Tib. 62.3) is alone preserving an alternative tradition that has Tiberius hating Gemellus as the product of adultery (probably between Sejanus and Livilla). Dio (58.23.2-4) presents the most balanced picture. He states that Tiberius determined that Gaius would succeed him, since he was older than Gemellus and, besides, Tiberius harbored doubts about Gemellus’s genealogy. Dio also says that Tiberius was sure that Gemellus would be killed, but did nothing to help him.
 Suet. Tib. 76. Philo (Leg.25, 28-29) and Dio (59.1.1-interpreted this act as a clear sign that Tiberius had intended Gemellus to succeed him, at the least in conjunction with Gaius; see also J.P.V.D. Balsdon, The Emperor Gaius (Oxford, 1934), 17-18. A joint accession, however, is most unlikely given the uncertainties of a new princeps’ political and legal position.
 Adoption and death: Philo Leg. 26-28; Suet. Gaius 15.2, 23.3; Dio 59.8.1; Jos. AJ 18.223. On the date of Gemellus’s death, see above, note . Gemellus’s laconic sepulchral inscription (“Here lies Tiberius Caesar, son of Drusus Caesar”) implies that his memory was not overly honoured, see Smallwood, Documents Illustrating, no. 88. Philo (Leg.30-31) preserves the longest account of his death, whereby the doomed youth was ordered to commit suicide under the supervision of Praetorian Guardsmen and, pathetically, had to be guided through the act by those more skilled in killing than himself. For a full and recent treatment, cf. A.A. Barrett, Caligula: The Corruption of Power (New Haven, 1989), 74-78.